diff --git a/test/client-connect.js b/test/client-connect.js
index 72e575035ef..7d9e561f242 100644
--- a/test/client-connect.js
+++ b/test/client-connect.js
@@ -13,7 +13,7 @@ test('connect aborted after connect', async (t) => {
   t = tspl(t, { plan: 3 })
 
   const signal = new EE()
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.fail()
   })
   server.on('connect', (req, c, firstBodyChunk) => {
diff --git a/test/client-head-reset-override.js b/test/client-head-reset-override.js
index 4582aa0a79c..d38e2991b8f 100644
--- a/test/client-head-reset-override.js
+++ b/test/client-head-reset-override.js
@@ -10,7 +10,7 @@ test('override HEAD reset', async (t) => {
   t = tspl(t, { plan: 4 })
 
   const expected = 'testing123'
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (req.method === 'GET') {
       res.write(expected)
     }
diff --git a/test/client-idempotent-body.js b/test/client-idempotent-body.js
index 6f40efdd9e0..434fe52f0db 100644
--- a/test/client-idempotent-body.js
+++ b/test/client-idempotent-body.js
@@ -9,7 +9,7 @@ test('idempotent retry', async (t) => {
   t = tspl(t, { plan: 11 })
 
   const body = 'world'
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     let buf = ''
     req.on('data', data => {
       buf += data
diff --git a/test/client-keep-alive.js b/test/client-keep-alive.js
index f00e4d8a128..e91f2ae0a0e 100644
--- a/test/client-keep-alive.js
+++ b/test/client-keep-alive.js
@@ -341,7 +341,7 @@ test('Disable keep alive', async (t) => {
   t = tspl(t, { plan: 7 })
 
   const ports = []
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(ports.includes(req.socket.remotePort), false)
     ports.push(req.socket.remotePort)
     t.strictEqual(req.headers.connection, 'close')
diff --git a/test/client-node-max-header-size.js b/test/client-node-max-header-size.js
index 2d1d8cbbcf2..d4461c35a1e 100644
--- a/test/client-node-max-header-size.js
+++ b/test/client-node-max-header-size.js
@@ -10,7 +10,7 @@ describe("Node.js' --max-http-header-size cli option", () => {
   let server
 
   before(async () => {
-    server = createServer((req, res) => {
+    server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(200, 'OK', {
         'Content-Length': 2
       })
diff --git a/test/client-pipeline.js b/test/client-pipeline.js
index bc2cd1d3a95..ec471a44b50 100644
--- a/test/client-pipeline.js
+++ b/test/client-pipeline.js
@@ -16,7 +16,7 @@ const {
 test('pipeline get', async (t) => {
   t = tspl(t, { plan: 17 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     t.strictEqual(`localhost:${server.address().port}`, req.headers.host)
@@ -75,7 +75,7 @@ test('pipeline get', async (t) => {
 test('pipeline echo', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   after(() => server.close())
@@ -124,7 +124,7 @@ test('pipeline ignore request body', async (t) => {
   t = tspl(t, { plan: 2 })
 
   let done
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write('asd')
     res.end()
     done()
@@ -185,7 +185,7 @@ test('pipeline invalid handler', async (t) => {
 test('pipeline invalid handler return after destroy should not error', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   after(() => server.close())
@@ -221,7 +221,7 @@ test('pipeline invalid handler return after destroy should not error', async (t)
 test('pipeline error body', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   after(() => server.close())
@@ -263,7 +263,7 @@ test('pipeline error body', async (t) => {
 test('pipeline destroy body', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   after(() => server.close())
@@ -305,7 +305,7 @@ test('pipeline destroy body', async (t) => {
 test('pipeline backpressure', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   after(() => server.close())
@@ -340,7 +340,7 @@ test('pipeline backpressure', async (t) => {
 test('pipeline invalid handler return', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   after(() => server.close())
@@ -381,7 +381,7 @@ test('pipeline invalid handler return', async (t) => {
 test('pipeline throw handler', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   after(() => server.close())
@@ -410,7 +410,7 @@ test('pipeline throw handler', async (t) => {
 test('pipeline destroy and throw handler', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   after(() => server.close())
@@ -444,7 +444,7 @@ test('pipeline abort res', async (t) => {
   t = tspl(t, { plan: 2 })
 
   let _res
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write('asd')
     _res = res
   })
@@ -483,7 +483,7 @@ test('pipeline abort res', async (t) => {
 test('pipeline abort server res', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.destroy()
   })
   after(() => server.close())
@@ -510,7 +510,7 @@ test('pipeline abort server res', async (t) => {
 test('pipeline abort duplex', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   after(() => server.close())
@@ -543,7 +543,7 @@ test('pipeline abort duplex', async (t) => {
 test('pipeline abort piped res', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write('asd')
   })
   after(() => server.close())
@@ -574,7 +574,7 @@ test('pipeline abort piped res', async (t) => {
 test('pipeline abort piped res 2', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write('asd')
   })
   after(() => server.close())
@@ -609,7 +609,7 @@ test('pipeline abort piped res 2', async (t) => {
 test('pipeline abort piped res 3', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write('asd')
   })
   after(() => server.close())
@@ -645,7 +645,7 @@ test('pipeline abort server res after headers', async (t) => {
   t = tspl(t, { plan: 1 })
 
   let _res
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write('asd')
     _res = res
   })
@@ -675,7 +675,7 @@ test('pipeline w/ write abort server res after headers', async (t) => {
   t = tspl(t, { plan: 1 })
 
   let _res
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
     _res = res
   })
@@ -706,7 +706,7 @@ test('destroy in push', async (t) => {
   t = tspl(t, { plan: 3 })
 
   let _res
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write('asd')
     _res = res
   })
@@ -763,7 +763,7 @@ test('pipeline args validation', async (t) => {
 test('pipeline factory throw not unhandled', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write('asd')
   })
   after(() => server.close())
@@ -790,7 +790,7 @@ test('pipeline factory throw not unhandled', async (t) => {
 test('pipeline destroy before dispatch', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('hello')
   })
   after(() => server.close())
@@ -816,7 +816,7 @@ test('pipeline destroy before dispatch', async (t) => {
 test('pipeline legacy stream', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write(Buffer.alloc(16e3))
     setImmediate(() => {
       res.end(Buffer.alloc(16e3))
@@ -847,7 +847,7 @@ test('pipeline legacy stream', async (t) => {
 test('pipeline objectMode', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify({ asd: 1 }))
   })
   after(() => server.close())
@@ -877,7 +877,7 @@ test('pipeline objectMode', async (t) => {
 test('pipeline invalid opts', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify({ asd: 1 }))
   })
   after(() => server.close())
@@ -904,7 +904,7 @@ test('pipeline invalid opts', async (t) => {
 test('pipeline CONNECT throw', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -932,7 +932,7 @@ test('pipeline CONNECT throw', async (t) => {
 test('pipeline body without destroy', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -962,7 +962,7 @@ test('pipeline body without destroy', async (t) => {
 test('pipeline ignore 1xx', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeProcessing()
     res.end('hello')
   })
@@ -993,7 +993,7 @@ test('pipeline ignore 1xx and use onInfo', async (t) => {
   t = tspl(t, { plan: 3 })
 
   const infos = []
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeProcessing()
     res.end('hello')
   })
@@ -1030,7 +1030,7 @@ test('pipeline backpressure', async (t) => {
 
   const expected = Buffer.alloc(1e6).toString()
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeProcessing()
     res.end(expected)
   })
@@ -1068,7 +1068,7 @@ test('pipeline backpressure', async (t) => {
 test('pipeline abort after headers', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeProcessing()
     res.write('asd')
     setImmediate(() => {
diff --git a/test/client-pipelining.js b/test/client-pipelining.js
index b62357f03e0..9e1281b53a3 100644
--- a/test/client-pipelining.js
+++ b/test/client-pipelining.js
@@ -16,7 +16,7 @@ test('20 times GET with pipelining 10', async (t) => {
 
   let count = 0
   let countGreaterThanOne = false
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     count++
     setTimeout(function () {
       countGreaterThanOne = countGreaterThanOne || count > 1
@@ -83,7 +83,7 @@ test('A client should enqueue as much as twice its pipelining factor', async (t)
 
   let count = 0
   let countGreaterThanOne = false
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     count++
     t.ok(count <= 5)
     setTimeout(function () {
@@ -137,7 +137,7 @@ test('pipeline 1 is 1 active request', async (t) => {
   t = tspl(t, { plan: 9 })
 
   let res2
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write('asd')
     res2 = res
   })
@@ -185,7 +185,7 @@ test('pipelined chunked POST stream', async (t) => {
   let a = 0
   let b = 0
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.on('data', chunk => {
       // Make sure a and b don't interleave.
       t.ok(a === 9 || b === 0)
@@ -254,7 +254,7 @@ test('pipelined chunked POST iterator', async (t) => {
   let a = 0
   let b = 0
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.on('data', chunk => {
       // Make sure a and b don't interleave.
       t.ok(a === 9 || b === 0)
@@ -322,7 +322,7 @@ function errordInflightPost (bodyType) {
     t = tspl(t, { plan: 6 })
 
     let serverRes
-    const server = createServer()
+    const server = createServer({ joinDuplicateHeaders: true })
     server.on('request', (req, res) => {
       serverRes = res
       res.write('asd')
@@ -380,7 +380,7 @@ errordInflightPost(consts.ASYNC_ITERATOR)
 test('pipelining non-idempotent', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.on('request', (req, res) => {
     setTimeout(() => {
       res.end('asd')
@@ -426,7 +426,7 @@ function pipeliningNonIdempotentWithBody (bodyType) {
   test(`pipelining non-idempotent w body ${bodyType}`, async (t) => {
     t = tspl(t, { plan: 4 })
 
-    const server = createServer()
+    const server = createServer({ joinDuplicateHeaders: true })
     server.on('request', (req, res) => {
       setImmediate(() => {
         res.end('asd')
@@ -489,7 +489,7 @@ function pipeliningHeadBusy (bodyType) {
   test(`pipelining HEAD busy ${bodyType}`, async (t) => {
     t = tspl(t, { plan: 7 })
 
-    const server = createServer()
+    const server = createServer({ joinDuplicateHeaders: true })
     server.on('request', (req, res) => {
       res.end('asd')
     })
@@ -561,7 +561,7 @@ test('pipelining empty pipeline before reset', async (t) => {
   t = tspl(t, { plan: 8 })
 
   let c = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.on('request', (req, res) => {
     if (c++ === 0) {
       res.end('asd')
@@ -624,7 +624,7 @@ function pipeliningIdempotentBusy (bodyType) {
   test(`pipelining idempotent busy ${bodyType}`, async (t) => {
     t = tspl(t, { plan: 12 })
 
-    const server = createServer()
+    const server = createServer({ joinDuplicateHeaders: true })
     server.on('request', (req, res) => {
       res.end('asd')
     })
@@ -728,7 +728,7 @@ pipeliningIdempotentBusy(consts.ASYNC_ITERATOR)
 test('pipelining blocked', async (t) => {
   t = tspl(t, { plan: 6 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
 
   let blocking = true
   let count = 0
diff --git a/test/client-post.js b/test/client-post.js
index e666faf0714..375f9c54b61 100644
--- a/test/client-post.js
+++ b/test/client-post.js
@@ -10,7 +10,7 @@ const { Blob } = require('node:buffer')
 test('request post blob', { skip: !Blob }, async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     t.strictEqual(req.headers['content-type'], 'application/json')
     let str = ''
     for await (const chunk of req) {
@@ -46,7 +46,7 @@ test('request post blob', { skip: !Blob }, async (t) => {
 test('request post arrayBuffer', { skip: !Blob }, async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     let str = ''
     for await (const chunk of req) {
       str += chunk
diff --git a/test/client-reconnect.js b/test/client-reconnect.js
index 222922a5ea0..7a1725da2b0 100644
--- a/test/client-reconnect.js
+++ b/test/client-reconnect.js
@@ -21,7 +21,7 @@ test('multiple reconnect', async (t) => {
     Object.assign(timers, orgTimers)
   })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     n === 0 ? res.destroy() : res.end('ok')
   })
   after(() => server.close())
diff --git a/test/client-request.js b/test/client-request.js
index 9712820b3a3..295d953ba7f 100644
--- a/test/client-request.js
+++ b/test/client-request.js
@@ -17,7 +17,7 @@ const { parseFormDataString } = require('./utils/formdata')
 test('request dump head', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-length', 5 * 100)
     res.flushHeaders()
     res.write('hello'.repeat(100))
@@ -50,7 +50,7 @@ test('request dump head', async (t) => {
 test('request dump big', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-length', 999999999)
     while (res.write('asd')) {
       // Do nothing...
@@ -85,7 +85,7 @@ test('request dump big', async (t) => {
 test('request dump', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.shouldKeepAlive = false
     res.setHeader('content-length', 5)
     res.end('hello')
@@ -117,7 +117,7 @@ test('request dump', async (t) => {
 
 test('request dump with abort signal', async (t) => {
   t = tspl(t, { plan: 2 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write('hello')
   })
   after(() => server.close())
@@ -145,7 +145,7 @@ test('request dump with abort signal', async (t) => {
 
 test('request hwm', async (t) => {
   t = tspl(t, { plan: 2 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write('hello')
   })
   after(() => server.close())
@@ -172,7 +172,7 @@ test('request abort before headers', async (t) => {
   t = tspl(t, { plan: 6 })
 
   const signal = new EE()
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('hello')
     signal.emit('abort')
   })
@@ -211,7 +211,7 @@ test('request abort before headers', async (t) => {
 test('request body destroyed on invalid callback', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
   })
   after(() => server.close())
 
@@ -239,7 +239,7 @@ test('request body destroyed on invalid callback', async (t) => {
 test('trailers', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200, { Trailer: 'Content-MD5' })
     res.addTrailers({ 'Content-MD5': 'test' })
     res.end()
@@ -268,7 +268,7 @@ test('trailers', async (t) => {
 test('destroy socket abruptly', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = net.createServer((socket) => {
+  const server = net.createServer({ joinDuplicateHeaders: true }, (socket) => {
     const lines = [
       'HTTP/1.1 200 OK',
       'Date: Sat, 09 Oct 2010 14:28:02 GMT',
@@ -309,7 +309,7 @@ test('destroy socket abruptly', async (t) => {
 test('destroy socket abruptly with keep-alive', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = net.createServer((socket) => {
+  const server = net.createServer({ joinDuplicateHeaders: true }, (socket) => {
     const lines = [
       'HTTP/1.1 200 OK',
       'Date: Sat, 09 Oct 2010 14:28:02 GMT',
@@ -355,7 +355,7 @@ test('request json', async (t) => {
   t = tspl(t, { plan: 1 })
 
   const obj = { asd: true }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify(obj))
   })
   after(() => server.close())
@@ -378,7 +378,7 @@ test('request long multibyte json', async (t) => {
   t = tspl(t, { plan: 1 })
 
   const obj = { asd: 'あ'.repeat(100000) }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify(obj))
   })
   after(() => server.close())
@@ -401,7 +401,7 @@ test('request text', async (t) => {
   t = tspl(t, { plan: 1 })
 
   const obj = { asd: true }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify(obj))
   })
   after(() => server.close())
@@ -484,7 +484,7 @@ describe('headers', () => {
 
   describe('array', () => {
     let serverAddress
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end(JSON.stringify(req.headers))
     })
 
@@ -524,7 +524,7 @@ describe('headers', () => {
 
   describe('host', () => {
     let serverAddress
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end(req.headers.host)
     })
 
@@ -583,7 +583,7 @@ test('request long multibyte text', async (t) => {
   t = tspl(t, { plan: 1 })
 
   const obj = { asd: 'あ'.repeat(100000) }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify(obj))
   })
   after(() => server.close())
@@ -606,7 +606,7 @@ test('request blob', async (t) => {
   t = tspl(t, { plan: 2 })
 
   const obj = { asd: true }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('Content-Type', 'application/json')
     res.end(JSON.stringify(obj))
   })
@@ -633,7 +633,7 @@ test('request arrayBuffer', async (t) => {
   t = tspl(t, { plan: 2 })
 
   const obj = { asd: true }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify(obj))
   })
   after(() => server.close())
@@ -659,7 +659,7 @@ test('request bytes', async (t) => {
   t = tspl(t, { plan: 2 })
 
   const obj = { asd: true }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify(obj))
   })
   after(() => server.close())
@@ -685,7 +685,7 @@ test('request body', async (t) => {
   t = tspl(t, { plan: 1 })
 
   const obj = { asd: true }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify(obj))
   })
   after(() => server.close())
@@ -712,7 +712,7 @@ test('request body', async (t) => {
 test('request post body no missing data', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     let ret = ''
     for await (const chunk of req) {
       ret += chunk
@@ -747,7 +747,7 @@ test('request post body no missing data', async (t) => {
 test('request post body no extra data handler', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     let ret = ''
     for await (const chunk of req) {
       ret += chunk
@@ -786,7 +786,7 @@ test('request post body no extra data handler', async (t) => {
 test('request with onInfo callback', async (t) => {
   t = tspl(t, { plan: 3 })
   const infos = []
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeProcessing()
     res.setHeader('Content-Type', 'application/json')
     res.end(JSON.stringify({ foo: 'bar' }))
@@ -814,7 +814,7 @@ test('request with onInfo callback but socket is destroyed before end of respons
   t = tspl(t, { plan: 5 })
   const infos = []
   let response
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     response = res
     res.writeProcessing()
   })
@@ -849,7 +849,7 @@ test('request onInfo callback headers parsing', async (t) => {
   t = tspl(t, { plan: 4 })
   const infos = []
 
-  const server = net.createServer((socket) => {
+  const server = net.createServer({ joinDuplicateHeaders: true }, (socket) => {
     const lines = [
       'HTTP/1.1 103 Early Hints',
       'Link: </style.css>; rel=preload; as=style',
@@ -885,7 +885,7 @@ test('request raw responseHeaders', async (t) => {
   t = tspl(t, { plan: 4 })
   const infos = []
 
-  const server = net.createServer((socket) => {
+  const server = net.createServer({ joinDuplicateHeaders: true }, (socket) => {
     const lines = [
       'HTTP/1.1 103 Early Hints',
       'Link: </style.css>; rel=preload; as=style',
@@ -922,7 +922,7 @@ test('request formData', async (t) => {
   t = tspl(t, { plan: 1 })
 
   const obj = { asd: true }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify(obj))
   })
   after(() => server.close())
@@ -951,7 +951,7 @@ test('request text2', async (t) => {
   t = tspl(t, { plan: 2 })
 
   const obj = { asd: true }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify(obj))
   })
   after(() => server.close())
@@ -985,7 +985,7 @@ test('request with FormData body', async (t) => {
   fd.set('key', 'value')
   fd.set('file', new Blob(['Hello, world!']), 'hello_world.txt')
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     const contentType = req.headers['content-type']
     // ensure we received a multipart/form-data header
     t.ok(/^multipart\/form-data; boundary=-+formdata-undici-0\d+$/.test(contentType))
@@ -1034,7 +1034,7 @@ test('request post body Buffer from string', async (t) => {
   t = tspl(t, { plan: 2 })
   const requestBody = Buffer.from('abcdefghijklmnopqrstuvwxyz')
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     let ret = ''
     for await (const chunk of req) {
       ret += chunk
@@ -1066,7 +1066,7 @@ test('request post body Buffer from buffer', async (t) => {
   const fullBuffer = new TextEncoder().encode('abcdefghijklmnopqrstuvwxyz')
   const requestBody = Buffer.from(fullBuffer.buffer, 8, 16)
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     let ret = ''
     for await (const chunk of req) {
       ret += chunk
@@ -1098,7 +1098,7 @@ test('request post body Uint8Array', async (t) => {
   const fullBuffer = new TextEncoder().encode('abcdefghijklmnopqrstuvwxyz')
   const requestBody = new Uint8Array(fullBuffer.buffer, 8, 16)
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     let ret = ''
     for await (const chunk of req) {
       ret += chunk
@@ -1130,7 +1130,7 @@ test('request post body Uint32Array', async (t) => {
   const fullBuffer = new TextEncoder().encode('abcdefghijklmnopqrstuvwxyz')
   const requestBody = new Uint32Array(fullBuffer.buffer, 8, 4)
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     let ret = ''
     for await (const chunk of req) {
       ret += chunk
@@ -1162,7 +1162,7 @@ test('request post body Float64Array', async (t) => {
   const fullBuffer = new TextEncoder().encode('abcdefghijklmnopqrstuvwxyz')
   const requestBody = new Float64Array(fullBuffer.buffer, 8, 2)
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     let ret = ''
     for await (const chunk of req) {
       ret += chunk
@@ -1194,7 +1194,7 @@ test('request post body BigUint64Array', async (t) => {
   const fullBuffer = new TextEncoder().encode('abcdefghijklmnopqrstuvwxyz')
   const requestBody = new BigUint64Array(fullBuffer.buffer, 8, 2)
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     let ret = ''
     for await (const chunk of req) {
       ret += chunk
@@ -1226,7 +1226,7 @@ test('request post body DataView', async (t) => {
   const fullBuffer = new TextEncoder().encode('abcdefghijklmnopqrstuvwxyz')
   const requestBody = new DataView(fullBuffer.buffer, 8, 16)
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     let ret = ''
     for await (const chunk of req) {
       ret += chunk
@@ -1258,7 +1258,7 @@ test('request multibyte json with setEncoding', async (t) => {
 
   const asd = Buffer.from('あいうえお')
   const data = JSON.stringify({ asd })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write(data.slice(0, 1))
     setTimeout(() => {
       res.write(data.slice(1))
@@ -1286,7 +1286,7 @@ test('request multibyte text with setEncoding', async (t) => {
   t = tspl(t, { plan: 1 })
 
   const data = Buffer.from('あいうえお')
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write(data.slice(0, 1))
     setTimeout(() => {
       res.write(data.slice(1))
@@ -1314,7 +1314,7 @@ test('request multibyte text with setEncoding', async (t) => {
   t = tspl(t, { plan: 1 })
 
   const data = Buffer.from('あいうえお')
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write(data.slice(0, 1))
     setTimeout(() => {
       res.write(data.slice(1))
@@ -1342,7 +1342,7 @@ test('#3736 - Aborted Response (without consuming body)', async (t) => {
   const plan = tspl(t, { plan: 1 })
 
   const controller = new AbortController()
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       res.writeHead(200, 'ok', {
         'content-type': 'text/plain'
diff --git a/test/client-stream.js b/test/client-stream.js
index ccdbedf1b09..8428ae990db 100644
--- a/test/client-stream.js
+++ b/test/client-stream.js
@@ -10,7 +10,7 @@ const EE = require('node:events')
 test('stream get', async (t) => {
   t = tspl(t, { plan: 9 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     t.strictEqual(`localhost:${server.address().port}`, req.headers.host)
@@ -53,7 +53,7 @@ test('stream get', async (t) => {
 test('stream promise get', async (t) => {
   t = tspl(t, { plan: 6 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     t.strictEqual(`localhost:${server.address().port}`, req.headers.host)
@@ -90,7 +90,7 @@ test('stream promise get', async (t) => {
 test('stream GET destroy res', async (t) => {
   t = tspl(t, { plan: 14 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     t.strictEqual(`localhost:${server.address().port}`, req.headers.host)
@@ -150,7 +150,7 @@ test('stream GET destroy res', async (t) => {
 test('stream GET remote destroy', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write('asd')
     setImmediate(() => {
       res.destroy()
@@ -195,7 +195,7 @@ test('stream GET remote destroy', async (t) => {
 test('stream response resume back pressure and non standard error', async (t) => {
   t = tspl(t, { plan: 5 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write(Buffer.alloc(1e3))
     setImmediate(() => {
       res.write(Buffer.alloc(1e7))
@@ -246,7 +246,7 @@ test('stream response resume back pressure and non standard error', async (t) =>
 test('stream waits only for writable side', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(Buffer.alloc(1e3))
   })
   after(() => server.close())
@@ -311,7 +311,7 @@ test('stream args validation promise', async (t) => {
 test('stream destroy if not readable', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   after(() => server.close())
@@ -339,7 +339,7 @@ test('stream destroy if not readable', async (t) => {
 test('stream server side destroy', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.destroy()
   })
   after(() => server.close())
@@ -364,7 +364,7 @@ test('stream server side destroy', async (t) => {
 test('stream invalid return', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write('asd')
   })
   after(() => server.close())
@@ -389,7 +389,7 @@ test('stream invalid return', async (t) => {
 test('stream body without destroy', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -417,7 +417,7 @@ test('stream body without destroy', async (t) => {
 test('stream factory abort', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -447,7 +447,7 @@ test('stream factory abort', async (t) => {
 test('stream factory throw', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -488,7 +488,7 @@ test('stream factory throw', async (t) => {
 test('stream CONNECT throw', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -512,7 +512,7 @@ test('stream CONNECT throw', async (t) => {
 test('stream abort after complete', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -541,7 +541,7 @@ test('stream abort after complete', async (t) => {
 test('stream abort before dispatch', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -570,7 +570,7 @@ test('stream abort before dispatch', async (t) => {
 test('trailers', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200, { Trailer: 'Content-MD5' })
     res.addTrailers({ 'Content-MD5': 'test' })
     res.end()
@@ -596,7 +596,7 @@ test('trailers', async (t) => {
 test('stream ignore 1xx', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeProcessing()
     res.end('hello')
   })
@@ -628,7 +628,7 @@ test('stream ignore 1xx and use onInfo', async (t) => {
   t = tspl(t, { plan: 4 })
 
   const infos = []
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeProcessing()
     res.end('hello')
   })
@@ -666,7 +666,7 @@ test('stream backpressure', async (t) => {
 
   const expected = Buffer.alloc(1e6).toString()
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeProcessing()
     res.end(expected)
   })
@@ -698,7 +698,7 @@ test('stream backpressure', async (t) => {
 test('stream body destroyed on invalid callback', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
   })
   after(() => server.close())
 
@@ -726,7 +726,7 @@ test('stream body destroyed on invalid callback', async (t) => {
 test('stream needDrain', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(Buffer.alloc(4096))
   })
   after(() => server.close())
@@ -782,7 +782,7 @@ test('stream needDrain', async (t) => {
 test('stream legacy needDrain', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(Buffer.alloc(4096))
   })
   after(() => server.close())
diff --git a/test/client-timeout.js b/test/client-timeout.js
index c4ff9c2a59c..26cf21913c2 100644
--- a/test/client-timeout.js
+++ b/test/client-timeout.js
@@ -11,7 +11,7 @@ const timers = require('../lib/util/timers')
 test('refresh timeout on pause', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.flushHeaders()
   })
   after(() => server.close())
@@ -61,7 +61,7 @@ test('start headers timeout after request body', async (t) => {
     Object.assign(timers, orgTimers)
   })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
   })
   after(() => server.close())
 
@@ -119,7 +119,7 @@ test('start headers timeout after async iterator request body', async (t) => {
     Object.assign(timers, orgTimers)
   })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
   })
   after(() => server.close())
 
@@ -169,7 +169,7 @@ test('start headers timeout after async iterator request body', async (t) => {
 test('parser resume with no body timeout', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
diff --git a/test/client-unref.js b/test/client-unref.js
index 49df4244e8e..ac25d636a8a 100644
--- a/test/client-unref.js
+++ b/test/client-unref.js
@@ -11,7 +11,7 @@ if (isMainThread) {
   test('client automatically closes itself when idle', async t => {
     t = tspl(t, { plan: 1 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end()
     })
     after(server.close.bind(server))
diff --git a/test/client-upgrade.js b/test/client-upgrade.js
index 7d76f8ee148..a1871a96501 100644
--- a/test/client-upgrade.js
+++ b/test/client-upgrade.js
@@ -11,7 +11,7 @@ const { kBusy } = require('../lib/core/symbols')
 test('basic upgrade', async (t) => {
   t = tspl(t, { plan: 6 })
 
-  const server = net.createServer((c) => {
+  const server = net.createServer({ joinDuplicateHeaders: true }, (c) => {
     c.on('data', (d) => {
       t.ok(/upgrade: websocket/i.test(d))
       c.write('HTTP/1.1 101\r\n')
@@ -70,7 +70,7 @@ test('basic upgrade', async (t) => {
 test('basic upgrade promise', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = net.createServer((c) => {
+  const server = net.createServer({ joinDuplicateHeaders: true }, (c) => {
     c.on('data', (d) => {
       c.write('HTTP/1.1 101\r\n')
       c.write('hello: world\r\n')
@@ -119,7 +119,7 @@ test('basic upgrade promise', async (t) => {
 test('upgrade error', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = net.createServer((c) => {
+  const server = net.createServer({ joinDuplicateHeaders: true }, (c) => {
     c.on('data', (d) => {
       c.write('HTTP/1.1 101\r\n')
       c.write('hello: world\r\n')
@@ -182,7 +182,7 @@ test('upgrade invalid opts', async (t) => {
 test('basic upgrade2', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = http.createServer()
+  const server = http.createServer({ joinDuplicateHeaders: true })
   server.on('upgrade', (req, c, head) => {
     c.write('HTTP/1.1 101\r\n')
     c.write('hello: world\r\n')
@@ -232,7 +232,7 @@ test('upgrade wait for empty pipeline', async (t) => {
   t = tspl(t, { plan: 7 })
 
   let canConnect = false
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
     canConnect = true
   })
@@ -299,7 +299,7 @@ test('upgrade wait for empty pipeline', async (t) => {
 test('upgrade aborted', async (t) => {
   t = tspl(t, { plan: 6 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.fail()
   })
   server.on('upgrade', (req, c, firstBodyChunk) => {
@@ -339,7 +339,7 @@ test('basic aborted after res', async (t) => {
   t = tspl(t, { plan: 1 })
 
   const signal = new EE()
-  const server = http.createServer()
+  const server = http.createServer({ joinDuplicateHeaders: true })
   server.on('upgrade', (req, c, head) => {
     c.write('HTTP/1.1 101\r\n')
     c.write('hello: world\r\n')
@@ -375,7 +375,7 @@ test('basic aborted after res', async (t) => {
 test('basic upgrade error', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = net.createServer((c) => {
+  const server = net.createServer({ joinDuplicateHeaders: true }, (c) => {
     c.on('data', (d) => {
       c.write('HTTP/1.1 101\r\n')
       c.write('hello: world\r\n')
@@ -445,7 +445,7 @@ test('upgrade disconnect', async (t) => {
 test('upgrade invalid signal', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = net.createServer(() => {
+  const server = net.createServer({ joinDuplicateHeaders: true }, () => {
     t.fail()
   })
   after(() => server.close())
diff --git a/test/client-write-max-listeners.js b/test/client-write-max-listeners.js
index c76ce60b0ab..09ab13cafb8 100644
--- a/test/client-write-max-listeners.js
+++ b/test/client-write-max-listeners.js
@@ -10,7 +10,7 @@ const { Readable } = require('node:stream')
 test('socket close listener does not leak', async (t) => {
   t = tspl(t, { plan: 32 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
 
   server.on('request', (req, res) => {
     res.end('hello')
diff --git a/test/client.js b/test/client.js
index 680ff66be10..6b031be173a 100644
--- a/test/client.js
+++ b/test/client.js
@@ -22,7 +22,7 @@ const hasIPv6 = (() => {
 test('basic get', async (t) => {
   t = tspl(t, { plan: 24 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     t.strictEqual(`localhost:${server.address().port}`, req.headers.host)
@@ -94,7 +94,7 @@ test('basic get', async (t) => {
 test('basic get with custom request.reset=true', async (t) => {
   t = tspl(t, { plan: 26 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     t.strictEqual(`localhost:${server.address().port}`, req.headers.host)
@@ -167,7 +167,7 @@ test('basic get with custom request.reset=true', async (t) => {
 test('basic get with query params', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     const searchParamsObject = buildParams(req.url)
     t.deepStrictEqual(searchParamsObject, {
       bool: 'true',
@@ -220,7 +220,7 @@ test('basic get with query params', async (t) => {
 test('basic get with query params fails if url includes hashmark', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.fail()
   })
   after(() => server.close())
@@ -254,7 +254,7 @@ test('basic get with query params fails if url includes hashmark', async (t) =>
 test('basic get with empty query params', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     const searchParamsObject = buildParams(req.url)
     t.deepStrictEqual(searchParamsObject, {})
 
@@ -291,7 +291,7 @@ test('basic get with empty query params', async (t) => {
 test('basic get with query params partially in path', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.fail()
   })
   after(() => server.close())
@@ -323,7 +323,7 @@ test('basic get with query params partially in path', async (t) => {
 test('using throwOnError should throw (request)', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.statusCode = 400
     res.end('hello')
   })
@@ -353,7 +353,7 @@ test('using throwOnError should throw (request)', async (t) => {
 test('using throwOnError should throw (stream)', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.statusCode = 400
     res.end('hello')
   })
@@ -387,7 +387,7 @@ test('using throwOnError should throw (stream)', async (t) => {
 test('basic head', async (t) => {
   t = tspl(t, { plan: 14 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/123', req.url)
     t.strictEqual('HEAD', req.method)
     t.strictEqual(`localhost:${server.address().port}`, req.headers.host)
@@ -429,7 +429,7 @@ test('basic head', async (t) => {
 test('basic head (IPv6)', { skip: !hasIPv6 }, async (t) => {
   t = tspl(t, { plan: 15 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/123', req.url)
     t.strictEqual('HEAD', req.method)
     t.strictEqual(`[::1]:${server.address().port}`, req.headers.host)
@@ -471,7 +471,7 @@ test('basic head (IPv6)', { skip: !hasIPv6 }, async (t) => {
 test('get with host header', async (t) => {
   t = tspl(t, { plan: 7 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     t.strictEqual('example.com', req.headers.host)
@@ -504,7 +504,7 @@ test('get with host header', async (t) => {
 test('get with host header (IPv6)', { skip: !hasIPv6 }, async (t) => {
   t = tspl(t, { plan: 7 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     t.strictEqual('[::1]', req.headers.host)
@@ -537,7 +537,7 @@ test('get with host header (IPv6)', { skip: !hasIPv6 }, async (t) => {
 test('head with host header', async (t) => {
   t = tspl(t, { plan: 7 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('HEAD', req.method)
     t.strictEqual('example.com', req.headers.host)
@@ -588,7 +588,7 @@ test('basic POST with string', async (t) => {
 
   const expected = readFileSync(__filename, 'utf8')
 
-  const server = createServer(postServer(t, expected))
+  const server = createServer({ joinDuplicateHeaders: true }, postServer(t, expected))
   after(() => server.close())
 
   server.listen(0, () => {
@@ -615,7 +615,7 @@ test('basic POST with string', async (t) => {
 test('basic POST with empty string', async (t) => {
   t = tspl(t, { plan: 7 })
 
-  const server = createServer(postServer(t, ''))
+  const server = createServer({ joinDuplicateHeaders: true }, postServer(t, ''))
   after(() => server.close())
 
   server.listen(0, () => {
@@ -643,7 +643,7 @@ test('basic POST with string and content-length', async (t) => {
 
   const expected = readFileSync(__filename, 'utf8')
 
-  const server = createServer(postServer(t, expected))
+  const server = createServer({ joinDuplicateHeaders: true }, postServer(t, expected))
   after(() => server.close())
 
   server.listen(0, () => {
@@ -678,7 +678,7 @@ test('basic POST with Buffer', async (t) => {
 
   const expected = readFileSync(__filename)
 
-  const server = createServer(postServer(t, expected.toString()))
+  const server = createServer({ joinDuplicateHeaders: true }, postServer(t, expected.toString()))
   after(() => server.close())
 
   server.listen(0, () => {
@@ -706,7 +706,7 @@ test('basic POST with stream', async (t) => {
 
   const expected = readFileSync(__filename, 'utf8')
 
-  const server = createServer(postServer(t, expected))
+  const server = createServer({ joinDuplicateHeaders: true }, postServer(t, expected))
   after(() => server.close())
 
   server.listen(0, () => {
@@ -742,7 +742,7 @@ test('basic POST with paused stream', async (t) => {
 
   const expected = readFileSync(__filename, 'utf8')
 
-  const server = createServer(postServer(t, expected))
+  const server = createServer({ joinDuplicateHeaders: true }, postServer(t, expected))
   after(() => server.close())
 
   server.listen(0, () => {
@@ -778,7 +778,7 @@ test('basic POST with paused stream', async (t) => {
 test('basic POST with custom stream', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.resume().on('end', () => {
       res.end('hello')
     })
@@ -835,7 +835,7 @@ test('basic POST with iterator', async (t) => {
 
   const expected = 'hello'
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.resume().on('end', () => {
       res.end(expected)
     })
@@ -879,7 +879,7 @@ test('basic POST with iterator', async (t) => {
 test('basic POST with iterator with invalid data', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer(() => {})
+  const server = createServer({ joinDuplicateHeaders: true }, () => {})
   after(() => server.close())
 
   const iterable = {
@@ -910,7 +910,7 @@ test('basic POST with async iterator', async (t) => {
 
   const expected = readFileSync(__filename, 'utf8')
 
-  const server = createServer(postServer(t, expected))
+  const server = createServer({ joinDuplicateHeaders: true }, postServer(t, expected))
   after(() => server.close())
 
   server.listen(0, () => {
@@ -945,7 +945,7 @@ test('basic POST with transfer encoding: chunked', async (t) => {
   t = tspl(t, { plan: 8 })
 
   let body
-  const server = createServer(function (req, res) {
+  const server = createServer({ joinDuplicateHeaders: true }, function (req, res) {
     t.strictEqual(req.url, '/')
     t.strictEqual(req.method, 'POST')
     t.strictEqual(req.headers['content-length'], undefined)
@@ -997,7 +997,7 @@ test('basic POST with transfer encoding: chunked', async (t) => {
 test('basic POST with empty stream', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer(function (req, res) {
+  const server = createServer({ joinDuplicateHeaders: true }, function (req, res) {
     t.deepStrictEqual(req.headers['content-length'], '0')
     req.pipe(res)
   })
@@ -1043,7 +1043,7 @@ test('10 times GET', async (t) => {
   const num = 10
   t = tspl(t, { plan: 3 * num })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(req.url)
   })
   after(() => server.close())
@@ -1078,7 +1078,7 @@ test('10 times HEAD', async (t) => {
   const num = 10
   t = tspl(t, { plan: num * 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(req.url)
   })
   after(() => server.close())
@@ -1110,7 +1110,7 @@ test('10 times HEAD', async (t) => {
 test('Set-Cookie', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.setHeader('Set-Cookie', ['a cookie', 'another cookie', 'more cookies'])
     res.end('hello')
@@ -1141,7 +1141,7 @@ test('Set-Cookie', async (t) => {
 test('ignore request header mutations', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.headers.test, 'test')
     res.end()
   })
@@ -1169,7 +1169,7 @@ test('ignore request header mutations', async (t) => {
 test('url-like url', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   after(() => server.close())
@@ -1196,7 +1196,7 @@ test('an absolute url as path', async (t) => {
 
   const path = 'http://example.com'
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.url, path)
     res.end()
   })
@@ -1222,7 +1222,7 @@ test('an absolute url as path', async (t) => {
 test('multiple destroy callback', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   after(() => server.close())
@@ -1257,7 +1257,7 @@ test('multiple destroy callback', async (t) => {
 test('only one streaming req at a time', async (t) => {
   t = tspl(t, { plan: 7 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   after(() => server.close())
@@ -1315,7 +1315,7 @@ test('only one streaming req at a time', async (t) => {
 test('only one async iterating req at a time', async (t) => {
   t = tspl(t, { plan: 6 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   after(() => server.close())
@@ -1371,7 +1371,7 @@ test('only one async iterating req at a time', async (t) => {
 test('300 requests succeed', async (t) => {
   t = tspl(t, { plan: 300 * 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -1431,7 +1431,7 @@ test('request args validation promise', async (t) => {
 test('increase pipelining', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.resume()
   })
   after(() => server.close())
@@ -1478,7 +1478,7 @@ test('destroy in push', async (t) => {
   t = tspl(t, { plan: 4 })
 
   let _res
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write('asd')
     _res = res
   })
@@ -1519,7 +1519,7 @@ test('destroy in push', async (t) => {
 test('non recoverable socket error fails pending request', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   after(() => server.close())
@@ -1545,7 +1545,7 @@ test('non recoverable socket error fails pending request', async (t) => {
 test('POST empty with error', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   after(() => server.close())
@@ -1576,7 +1576,7 @@ test('POST empty with error', async (t) => {
 test('busy', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   after(() => server.close())
@@ -1604,7 +1604,7 @@ test('busy', async (t) => {
 test('connected', async (t) => {
   t = tspl(t, { plan: 7 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     // needed so that disconnect is emitted
     res.setHeader('connection', 'close')
     req.pipe(res)
@@ -1645,7 +1645,7 @@ test('connected', async (t) => {
 test('emit disconnect after destroy', async t => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   after(() => server.close())
@@ -1674,7 +1674,7 @@ test('emit disconnect after destroy', async t => {
 test('end response before request', async t => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   after(() => server.close())
@@ -1709,7 +1709,7 @@ test('end response before request', async t => {
 
 test('parser pause with no body timeout', async (t) => {
   t = tspl(t, { plan: 2 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     let counter = 0
     const t = setInterval(() => {
       counter++
@@ -1742,7 +1742,7 @@ test('parser pause with no body timeout', async (t) => {
 
 test('TypedArray and DataView body', async (t) => {
   t = tspl(t, { plan: 3 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.headers['content-length'], '8')
     res.end()
   })
@@ -1768,7 +1768,7 @@ test('TypedArray and DataView body', async (t) => {
 test('async iterator empty chunk continues', async (t) => {
   t = tspl(t, { plan: 5 })
   const serverChunks = ['hello', 'world']
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     let str = ''
     let i = 0
     req.on('data', (chunk) => {
@@ -1805,7 +1805,7 @@ test('async iterator empty chunk continues', async (t) => {
 
 test('async iterator error from server destroys early', async (t) => {
   t = tspl(t, { plan: 3 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.on('data', (chunk) => {
       res.destroy()
     })
@@ -1843,7 +1843,7 @@ test('async iterator error from server destroys early', async (t) => {
 
 test('regular iterator error from server closes early', async (t) => {
   t = tspl(t, { plan: 3 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.on('data', () => {
       process.nextTick(() => {
         res.destroy()
@@ -1884,7 +1884,7 @@ test('regular iterator error from server closes early', async (t) => {
 
 test('async iterator early return closes early', async (t) => {
   t = tspl(t, { plan: 3 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.on('data', () => {
       res.writeHead(200)
       res.end()
@@ -1924,7 +1924,7 @@ test('async iterator yield unsupported TypedArray', {
   skip: !!require('stream')._isArrayBufferView
 }, async (t) => {
   t = tspl(t, { plan: 3 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.on('end', () => {
       res.writeHead(200)
       res.end()
@@ -1956,7 +1956,7 @@ test('async iterator yield unsupported TypedArray', {
 
 test('async iterator yield object error', async (t) => {
   t = tspl(t, { plan: 3 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.on('end', () => {
       res.writeHead(200)
       res.end()
@@ -1988,7 +1988,7 @@ test('async iterator yield object error', async (t) => {
 
 test('Successfully get a Response when neither a Transfer-Encoding or Content-Length header is present', async (t) => {
   t = tspl(t, { plan: 4 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.on('data', (data) => {
     })
     req.on('end', () => {
diff --git a/test/close-and-destroy.js b/test/close-and-destroy.js
index 5d4f135dcd2..e52c0072553 100644
--- a/test/close-and-destroy.js
+++ b/test/close-and-destroy.js
@@ -9,7 +9,7 @@ const { kSocket, kSize } = require('../lib/core/symbols')
 test('close waits for queued requests to finish', async (t) => {
   t = tspl(t, { plan: 16 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
 
   server.on('request', (req, res) => {
     t.ok(true, 'request received')
@@ -54,7 +54,7 @@ test('close waits for queued requests to finish', async (t) => {
 test('destroy invoked all pending callbacks', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
 
   server.on('request', (req, res) => {
     res.write('hello')
@@ -88,7 +88,7 @@ test('destroy invoked all pending callbacks', async (t) => {
 test('destroy invoked all pending callbacks ticked', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
 
   server.on('request', (req, res) => {
     res.write('hello')
@@ -120,7 +120,7 @@ test('destroy invoked all pending callbacks ticked', async (t) => {
 test('close waits until socket is destroyed', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(req.url)
   })
   after(() => server.close())
@@ -157,7 +157,7 @@ test('close waits until socket is destroyed', async (t) => {
 test('close should still reconnect', async (t) => {
   t = tspl(t, { plan: 6 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(req.url)
   })
   after(() => server.close())
@@ -192,7 +192,7 @@ test('close should still reconnect', async (t) => {
 test('close should call callback once finished', async (t) => {
   t = tspl(t, { plan: 6 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setImmediate(function () {
       res.end(req.url)
     })
@@ -266,7 +266,7 @@ test('close after and destroy should error', async (t) => {
 test('close socket and reconnect after maxRequestsPerClient reached', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(req.url)
   })
 
@@ -300,7 +300,7 @@ test('close socket and reconnect after maxRequestsPerClient reached', async (t)
 test('close socket and reconnect after maxRequestsPerClient reached (async)', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(req.url)
   })
 
@@ -336,7 +336,7 @@ test('close socket and reconnect after maxRequestsPerClient reached (async)', as
 test('should not close socket when no maxRequestsPerClient is provided', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(req.url)
   })
 
diff --git a/test/connect-pre-shared-session.js b/test/connect-pre-shared-session.js
index 5ee7b308885..c10e3c82a3d 100644
--- a/test/connect-pre-shared-session.js
+++ b/test/connect-pre-shared-session.js
@@ -12,7 +12,7 @@ test('custom session passed to client will be used in tls connect call', async (
 
   const mockConnect = mock.method(tls, 'connect')
 
-  const server = createServer(pem, (req, res) => {
+  const server = createServer({ ...pem, joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     res.setHeader('content-type', 'text/plain')
diff --git a/test/content-length.js b/test/content-length.js
index ece0747406a..6d8e7799c91 100644
--- a/test/content-length.js
+++ b/test/content-length.js
@@ -10,7 +10,7 @@ const { maybeWrapStream, consts } = require('./utils/async-iterators')
 test('request invalid content-length', async (t) => {
   t = tspl(t, { plan: 7 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   after(() => server.close())
@@ -103,7 +103,7 @@ function invalidContentLength (bodyType) {
   test(`request streaming ${bodyType} invalid content-length`, async (t) => {
     t = tspl(t, { plan: 4 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end()
     })
     after(() => server.close())
@@ -165,7 +165,7 @@ function zeroContentLength (bodyType) {
   test(`request ${bodyType} streaming data when content-length=0`, async (t) => {
     t = tspl(t, { plan: 1 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end()
     })
     after(() => server.close())
@@ -201,7 +201,7 @@ zeroContentLength(consts.ASYNC_ITERATOR)
 test('request streaming no body data when content-length=0', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   after(() => server.close())
@@ -233,7 +233,7 @@ test('request streaming no body data when content-length=0', async (t) => {
 test('response invalid content length with close', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200, {
       'content-length': 10
     })
@@ -270,7 +270,7 @@ test('response invalid content length with close', async (t) => {
 })
 
 test('request streaming with Readable.from(buf)', async (t) => {
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   after(() => server.close())
@@ -302,7 +302,7 @@ test('request streaming with Readable.from(buf)', async (t) => {
 
 test('request DELETE, content-length=0, with body', async (t) => {
   t = tspl(t, { plan: 5 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.shouldKeepAlive = false
     res.end()
   })
@@ -352,7 +352,7 @@ test('request DELETE, content-length=0, with body', async (t) => {
 test('content-length shouldSendContentLength=false', async (t) => {
   t = tspl(t, { plan: 15 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   server.on('request', (req, res) => {
diff --git a/test/env-http-proxy-agent-nodejs-bundle.js b/test/env-http-proxy-agent-nodejs-bundle.js
index 43746a318a5..f9f8d526d10 100644
--- a/test/env-http-proxy-agent-nodejs-bundle.js
+++ b/test/env-http-proxy-agent-nodejs-bundle.js
@@ -26,13 +26,13 @@ describe('EnvHttpProxyAgent and setGlobalDispatcher', () => {
     // Instead of using mocks, start a real server and a minimal proxy server
     // in order to exercise the actual paths in EnvHttpProxyAgent from the
     // Node.js bundle.
-    const server = http.createServer((req, res) => { res.end('Hello world') })
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => { res.end('Hello world') })
     server.on('error', err => { console.log('Server error', err) })
     server.listen(0)
     await once(server, 'listening')
     t.after(() => { server.close() })
 
-    const proxy = http.createServer()
+    const proxy = http.createServer({ joinDuplicateHeaders: true })
     proxy.on('connect', (req, clientSocket, head) => {
       // Check that the proxy is actually used to tunnel the request sent below.
       const [hostname, port] = req.url.split(':')
diff --git a/test/eventsource/eventsource-attributes.js b/test/eventsource/eventsource-attributes.js
index 0e046affdbd..42465c912c7 100644
--- a/test/eventsource/eventsource-attributes.js
+++ b/test/eventsource/eventsource-attributes.js
@@ -7,7 +7,7 @@ const { test, describe } = require('node:test')
 const { EventSource } = require('../../lib/web/eventsource/eventsource')
 
 describe('EventSource - eventhandler idl', async () => {
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200, 'dummy')
   })
 
diff --git a/test/eventsource/eventsource-close.js b/test/eventsource/eventsource-close.js
index 5b6397df63a..aef42e95185 100644
--- a/test/eventsource/eventsource-close.js
+++ b/test/eventsource/eventsource-close.js
@@ -9,7 +9,7 @@ const { EventSource } = require('../../lib/web/eventsource/eventsource')
 
 describe('EventSource - close', () => {
   test('should not emit error when closing the EventSource Instance', async () => {
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       assert.strictEqual(req.headers.connection, 'keep-alive')
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.write('data: hello\n\n')
@@ -32,7 +32,7 @@ describe('EventSource - close', () => {
   })
 
   test('should set readyState to CLOSED', async () => {
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       assert.strictEqual(req.headers.connection, 'keep-alive')
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.write('data: hello\n\n')
diff --git a/test/eventsource/eventsource-connect.js b/test/eventsource/eventsource-connect.js
index 75508ee954d..018032044fa 100644
--- a/test/eventsource/eventsource-connect.js
+++ b/test/eventsource/eventsource-connect.js
@@ -8,7 +8,7 @@ const { EventSource } = require('../../lib/web/eventsource/eventsource')
 
 describe('EventSource - sending correct request headers', () => {
   test('should send request with connection keep-alive', async () => {
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       assert.strictEqual(req.headers.connection, 'keep-alive')
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.end()
@@ -30,7 +30,7 @@ describe('EventSource - sending correct request headers', () => {
   })
 
   test('should send request with sec-fetch-mode set to cors', async () => {
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       assert.strictEqual(req.headers['sec-fetch-mode'], 'cors')
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.end()
@@ -52,7 +52,7 @@ describe('EventSource - sending correct request headers', () => {
   })
 
   test('should send request with pragma and cache-control set to no-cache', async () => {
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       assert.strictEqual(req.headers['cache-control'], 'no-cache')
       assert.strictEqual(req.headers.pragma, 'no-cache')
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
@@ -75,7 +75,7 @@ describe('EventSource - sending correct request headers', () => {
   })
 
   test('should send request with accept text/event-stream', async () => {
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       assert.strictEqual(req.headers.accept, 'text/event-stream')
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.end()
@@ -99,7 +99,7 @@ describe('EventSource - sending correct request headers', () => {
 
 describe('EventSource - received response must have content-type to be text/event-stream', () => {
   test('should send request with accept text/event-stream', async () => {
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.end()
     })
@@ -120,7 +120,7 @@ describe('EventSource - received response must have content-type to be text/even
   })
 
   test('should send request with accept text/event-stream;', async () => {
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream;' })
       res.end()
     })
@@ -141,7 +141,7 @@ describe('EventSource - received response must have content-type to be text/even
   })
 
   test('should handle content-type text/event-stream;charset=UTF-8 properly', async () => {
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream;charset=UTF-8' })
       res.end()
     })
@@ -162,7 +162,7 @@ describe('EventSource - received response must have content-type to be text/even
   })
 
   test('should throw if content-type is text/html properly', async () => {
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/html' })
       res.end()
     })
diff --git a/test/eventsource/eventsource-constructor-stringify.js b/test/eventsource/eventsource-constructor-stringify.js
index aee1d02bc6d..a8cd6b14986 100644
--- a/test/eventsource/eventsource-constructor-stringify.js
+++ b/test/eventsource/eventsource-constructor-stringify.js
@@ -8,7 +8,7 @@ const { EventSource } = require('../../lib/web/eventsource/eventsource')
 
 describe('EventSource - constructor stringify', () => {
   test('should stringify argument', async () => {
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       assert.strictEqual(req.headers.connection, 'keep-alive')
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.end()
diff --git a/test/eventsource/eventsource-constructor.js b/test/eventsource/eventsource-constructor.js
index a5e25e3f1dc..49a818e4cce 100644
--- a/test/eventsource/eventsource-constructor.js
+++ b/test/eventsource/eventsource-constructor.js
@@ -8,7 +8,7 @@ const { EventSource } = require('../../lib/web/eventsource/eventsource')
 
 describe('EventSource - withCredentials', () => {
   test('withCredentials should be false by default', async () => {
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.end()
     })
@@ -30,7 +30,7 @@ describe('EventSource - withCredentials', () => {
   })
 
   test('withCredentials can be set to true', async () => {
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.end()
     })
diff --git a/test/eventsource/eventsource-custom-dispatcher.js b/test/eventsource/eventsource-custom-dispatcher.js
index e04c2f80673..4a852a56e7b 100644
--- a/test/eventsource/eventsource-custom-dispatcher.js
+++ b/test/eventsource/eventsource-custom-dispatcher.js
@@ -9,7 +9,7 @@ const { test } = require('node:test')
 test('EventSource allows setting custom dispatcher.', async (t) => {
   const { completed, deepStrictEqual } = tspl(t, { plan: 1 })
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
     deepStrictEqual(req.headers['x-customer-header'], 'hello world')
 
diff --git a/test/eventsource/eventsource-message.js b/test/eventsource/eventsource-message.js
index d7843bc6891..9de1eae4b43 100644
--- a/test/eventsource/eventsource-message.js
+++ b/test/eventsource/eventsource-message.js
@@ -20,7 +20,7 @@ describe('EventSource - message', () => {
       finishedPromise.reject = reject
     })
 
-    const server = http.createServer(async (req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, async (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.write('retry: 100\n\n')
       await setTimeout(100)
@@ -66,7 +66,7 @@ describe('EventSource - message', () => {
       finishedPromise.reject = reject
     })
 
-    const server = http.createServer(async (req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, async (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.write('event:message\n\n')
       await setTimeout(100)
@@ -106,7 +106,7 @@ describe('EventSource - message', () => {
       finishedPromise.reject = reject
     })
 
-    const server = http.createServer(async (req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, async (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.write('event:custom\ndata:test\n\n')
       await setTimeout(100)
@@ -143,7 +143,7 @@ describe('EventSource - message', () => {
       finishedPromise.reject = reject
     })
 
-    const server = http.createServer(async (req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, async (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.write('data:test\n\n')
       await setTimeout(100)
@@ -180,7 +180,7 @@ describe('EventSource - message', () => {
       finishedPromise.reject = reject
     })
 
-    const server = http.createServer(async (req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, async (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.write('data\n\n')
       await setTimeout(100)
@@ -217,7 +217,7 @@ describe('EventSource - message', () => {
       finishedPromise.reject = reject
     })
 
-    const server = http.createServer(async (req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, async (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.write('event:custom\ndata:\n\n')
       await setTimeout(100)
@@ -254,7 +254,7 @@ describe('EventSource - message', () => {
       finishedPromise.reject = reject
     })
 
-    const server = http.createServer(async (req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, async (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.write('data:\n\n')
       await setTimeout(100)
@@ -291,7 +291,7 @@ describe('EventSource - message', () => {
       finishedPromise.reject = reject
     })
 
-    const server = http.createServer(async (req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, async (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.write('event:custom\ndata\n\n')
       await setTimeout(100)
@@ -328,7 +328,7 @@ describe('EventSource - message', () => {
       finishedPromise.reject = reject
     })
 
-    const server = http.createServer(async (req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, async (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.write('event:custom\n\n')
       await setTimeout(100)
diff --git a/test/eventsource/eventsource-reconnect.js b/test/eventsource/eventsource-reconnect.js
index 50d5f6dadec..8459bb00d69 100644
--- a/test/eventsource/eventsource-reconnect.js
+++ b/test/eventsource/eventsource-reconnect.js
@@ -19,7 +19,7 @@ describe('EventSource - reconnect', () => {
       finishedPromise.reject = reject
     })
 
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.end()
     })
@@ -52,7 +52,7 @@ describe('EventSource - reconnect', () => {
       finishedPromise.reject = reject
     })
 
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.end()
     })
@@ -87,7 +87,7 @@ describe('EventSource - reconnect', () => {
       finishedPromise.reject = reject
     })
 
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.write('retry: 100\n\n')
       res.end()
@@ -126,7 +126,7 @@ describe('EventSource - reconnect', () => {
       finishedPromise.reject = reject
     })
 
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(200, 'OK', { 'Content-Type': 'text/event-stream' })
       res.write('id: 1337\n\n')
       if (requestCount++ !== 0) {
diff --git a/test/eventsource/eventsource-redirecting.js b/test/eventsource/eventsource-redirecting.js
index 07bd36ea790..20bfa983994 100644
--- a/test/eventsource/eventsource-redirecting.js
+++ b/test/eventsource/eventsource-redirecting.js
@@ -9,7 +9,7 @@ const { EventSource } = require('../../lib/web/eventsource/eventsource')
 describe('EventSource - redirecting', () => {
   [301, 302, 307, 308].forEach((statusCode) => {
     test(`Should redirect on ${statusCode} status code`, async () => {
-      const server = http.createServer((req, res) => {
+      const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
         if (res.req.url === '/redirect') {
           res.writeHead(statusCode, undefined, { Location: '/target' })
           res.end()
@@ -37,7 +37,7 @@ describe('EventSource - redirecting', () => {
   })
 
   test('Stop trying to connect when getting a 204 response', async () => {
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       if (res.req.url === '/redirect') {
         res.writeHead(301, undefined, { Location: '/target' })
         res.end()
@@ -63,7 +63,7 @@ describe('EventSource - redirecting', () => {
   })
 
   test('Throw when missing a Location header', async () => {
-    const server = http.createServer((req, res) => {
+    const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       if (res.req.url === '/redirect') {
         res.writeHead(301, undefined)
         res.end()
@@ -86,7 +86,7 @@ describe('EventSource - redirecting', () => {
   })
 
   test('Should set origin attribute of messages after redirecting', async () => {
-    const targetServer = http.createServer((req, res) => {
+    const targetServer = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       if (res.req.url === '/target') {
         res.writeHead(200, undefined, { 'Content-Type': 'text/event-stream' })
         res.write('event: message\ndata: test\n\n')
@@ -96,7 +96,7 @@ describe('EventSource - redirecting', () => {
     await events.once(targetServer, 'listening')
     const targetPort = targetServer.address().port
 
-    const sourceServer = http.createServer((req, res) => {
+    const sourceServer = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(301, undefined, { Location: `http://127.0.0.1:${targetPort}/target` })
       res.end()
     })
diff --git a/test/eventsource/eventsource-request-status-error.js b/test/eventsource/eventsource-request-status-error.js
index 3b73d22b851..694ae160213 100644
--- a/test/eventsource/eventsource-request-status-error.js
+++ b/test/eventsource/eventsource-request-status-error.js
@@ -9,7 +9,7 @@ const { EventSource } = require('../../lib/web/eventsource/eventsource')
 describe('EventSource - status error', () => {
   [204, 205, 210, 299, 404, 410, 503].forEach((statusCode) => {
     test(`Should error on ${statusCode} status code`, async () => {
-      const server = http.createServer((req, res) => {
+      const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
         res.writeHead(statusCode, 'dummy', { 'Content-Type': 'text/event-stream' })
         res.end()
       })
diff --git a/test/examples.js b/test/examples.js
index e06b2d4ddb8..055f082b3fd 100644
--- a/test/examples.js
+++ b/test/examples.js
@@ -10,7 +10,7 @@ test('request examples', async (t) => {
   t = tspl(t, { plan: 7 })
 
   let lastReq
-  const exampleServer = createServer((req, res) => {
+  const exampleServer = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     lastReq = req
     if (req.method === 'DELETE') {
       res.statusCode = 204
@@ -29,7 +29,7 @@ test('request examples', async (t) => {
     }
   })
 
-  const errorServer = createServer((req, res) => {
+  const errorServer = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     lastReq = req
     res.statusCode = 400
     res.setHeader('content-type', 'application/json')
diff --git a/test/fetch/407-statuscode-window-null.js b/test/fetch/407-statuscode-window-null.js
index e03042e4c09..26e763c59dc 100644
--- a/test/fetch/407-statuscode-window-null.js
+++ b/test/fetch/407-statuscode-window-null.js
@@ -9,7 +9,7 @@ const assert = require('node:assert')
 const { closeServerAsPromise } = require('../utils/node-http')
 
 test('Receiving a 407 status code w/ a window option present should reject', async (t) => {
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.statusCode = 407
     res.end()
   }).listen(0)
diff --git a/test/fetch/abort.js b/test/fetch/abort.js
index 73b4764fe81..8f4a1f321b6 100644
--- a/test/fetch/abort.js
+++ b/test/fetch/abort.js
@@ -9,7 +9,7 @@ const { once } = require('node:events')
 const { closeServerAsPromise } = require('../utils/node-http')
 
 test('allows aborting with custom errors', async (t) => {
-  const server = createServer().listen(0)
+  const server = createServer({ joinDuplicateHeaders: true }).listen(0)
 
   t.after(closeServerAsPromise(server))
   await once(server, 'listening')
diff --git a/test/fetch/abort2.js b/test/fetch/abort2.js
index fec300b43df..f62e5c8c49a 100644
--- a/test/fetch/abort2.js
+++ b/test/fetch/abort2.js
@@ -16,7 +16,7 @@ test('parallel fetch with the same AbortController works as expected', async (t)
     bug: 'Ensure request is not aborted before enqueueing bytes into stream.'
   }
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.statusCode = 200
     res.end(JSON.stringify(body))
   })
diff --git a/test/fetch/client-fetch.js b/test/fetch/client-fetch.js
index 3003021390b..353a72aacf9 100644
--- a/test/fetch/client-fetch.js
+++ b/test/fetch/client-fetch.js
@@ -40,7 +40,7 @@ test('request json', (t, done) => {
   const { deepStrictEqual } = tspl(t, { plan: 1 })
 
   const obj = { asd: true }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify(obj))
   })
   t.after(closeServerAsPromise(server))
@@ -56,7 +56,7 @@ test('request text', (t, done) => {
   const { strictEqual } = tspl(t, { plan: 1 })
 
   const obj = { asd: true }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify(obj))
   })
   t.after(closeServerAsPromise(server))
@@ -72,7 +72,7 @@ test('request arrayBuffer', (t, done) => {
   const { deepStrictEqual } = tspl(t, { plan: 1 })
 
   const obj = { asd: true }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify(obj))
   })
   t.after(closeServerAsPromise(server))
@@ -88,7 +88,7 @@ test('should set type of blob object to the value of the `Content-Type` header f
   const { strictEqual } = tspl(t, { plan: 1 })
 
   const obj = { asd: true }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('Content-Type', 'application/json')
     res.end(JSON.stringify(obj))
   })
@@ -104,7 +104,7 @@ test('should set type of blob object to the value of the `Content-Type` header f
 test('pre aborted with readable request body', (t, done) => {
   const { strictEqual } = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
   })
   t.after(closeServerAsPromise(server))
 
@@ -129,7 +129,7 @@ test('pre aborted with readable request body', (t, done) => {
 test('pre aborted with closed readable request body', (t, done) => {
   const { ok, strictEqual } = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
   })
   t.after(closeServerAsPromise(server))
 
@@ -161,7 +161,7 @@ test('pre aborted with closed readable request body', (t, done) => {
 test('unsupported formData 1', (t, done) => {
   const { strictEqual } = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'asdasdsad')
     res.end()
   })
@@ -190,7 +190,7 @@ test('multipart formdata not base64', async (t) => {
   const boundary = tempRes.headers.get('content-type').split('boundary=')[1]
   const formRaw = await tempRes.text()
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'multipart/form-data; boundary=' + boundary)
     res.write(formRaw)
     res.end()
@@ -223,7 +223,7 @@ test('multipart formdata base64', (t, done) => {
     '\r\n' +
     '------formdata-undici-0.5786922755719377--'
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     res.setHeader('content-type', 'multipart/form-data; boundary=----formdata-undici-0.5786922755719377')
 
     for (let offset = 0; offset < formRaw.length;) {
@@ -274,7 +274,7 @@ test('busboy emit error', async (t) => {
   const tempRes = new Response(formData)
   const formRaw = await tempRes.text()
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'multipart/form-data; boundary=wrongboundary')
     res.write(formRaw)
     res.end()
@@ -303,7 +303,7 @@ test('parsing formData preserve full path on files', async (t) => {
 test('urlencoded formData', (t, done) => {
   const { strictEqual } = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'application/x-www-form-urlencoded')
     res.end('field1=value1&field2=value2')
   })
@@ -323,7 +323,7 @@ test('urlencoded formData', (t, done) => {
 test('text with BOM', (t, done) => {
   const { strictEqual } = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'application/x-www-form-urlencoded')
     res.end('\uFEFFtest=\uFEFF')
   })
@@ -342,7 +342,7 @@ test('text with BOM', (t, done) => {
 test('formData with BOM', (t, done) => {
   const { strictEqual } = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'application/x-www-form-urlencoded')
     res.end('\uFEFFtest=\uFEFF')
   })
@@ -361,7 +361,7 @@ test('formData with BOM', (t, done) => {
 test('locked blob body', (t, done) => {
   const { strictEqual } = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   t.after(closeServerAsPromise(server))
@@ -379,7 +379,7 @@ test('locked blob body', (t, done) => {
 test('disturbed blob body', (t, done) => {
   const { ok, strictEqual } = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   t.after(closeServerAsPromise(server))
@@ -400,7 +400,7 @@ test('redirect with body', (t, done) => {
   const { strictEqual } = tspl(t, { plan: 3 })
 
   let count = 0
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     let body = ''
     for await (const chunk of req) {
       body += chunk
@@ -431,7 +431,7 @@ test('redirect with stream', (t, done) => {
 
   const location = '/asd'
   const body = 'hello!'
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     res.writeHead(302, { location })
     let count = 0
     const l = setInterval(() => {
@@ -493,7 +493,7 @@ test('post FormData with Blob', (t, done) => {
   const body = new FormData()
   body.append('field1', new Blob(['asd1']))
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   t.after(closeServerAsPromise(server))
@@ -514,7 +514,7 @@ test('post FormData with File', (t, done) => {
   const body = new FormData()
   body.append('field1', new File(['asd1'], 'filename123'))
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   })
   t.after(closeServerAsPromise(server))
@@ -545,7 +545,7 @@ test('custom agent', (t, done) => {
   const { ok, deepStrictEqual } = tspl(t, { plan: 2 })
 
   const obj = { asd: true }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify(obj))
   })
   t.after(closeServerAsPromise(server))
@@ -572,7 +572,7 @@ test('custom agent node fetch', (t, done) => {
   const { ok, deepStrictEqual } = tspl(t, { plan: 2 })
 
   const obj = { asd: true }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify(obj))
   })
   t.after(closeServerAsPromise(server))
@@ -596,7 +596,7 @@ test('custom agent node fetch', (t, done) => {
 })
 
 test('error on redirect', (t, done) => {
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.statusCode = 302
     res.end()
   })
@@ -614,7 +614,7 @@ test('error on redirect', (t, done) => {
 
 // https://github.com/nodejs/undici/issues/1527
 test('fetching with Request object - issue #1527', async (t) => {
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     assert.ok(true)
     res.end()
   }).listen(0)
@@ -635,7 +635,7 @@ test('do not decode redirect body', (t, done) => {
   const { ok, strictEqual } = tspl(t, { plan: 3 })
 
   const obj = { asd: true }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (req.url === '/resource') {
       ok(true)
       res.statusCode = 301
@@ -663,7 +663,7 @@ test('decode non-redirect body with location header', (t, done) => {
   const { ok, strictEqual } = tspl(t, { plan: 2 })
 
   const obj = { asd: true }
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     ok(true)
     res.statusCode = 201
     res.setHeader('location', '/resource/')
@@ -688,7 +688,7 @@ test('Receiving non-Latin1 headers', async (t) => {
     'inline; filename="100 % loading&perf.png"; filename*=UTF-8\'\'100%20%25%20loading%26perf.png'
   ]
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     for (let i = 0; i < ContentDisposition.length; i++) {
       res.setHeader(`Content-Disposition-${i + 1}`, ContentDisposition[i])
     }
diff --git a/test/fetch/client-node-max-header-size.js b/test/fetch/client-node-max-header-size.js
index 65d9251dc03..533d2377737 100644
--- a/test/fetch/client-node-max-header-size.js
+++ b/test/fetch/client-node-max-header-size.js
@@ -10,7 +10,7 @@ describe('fetch respects --max-http-header-size', () => {
   let server
 
   before(async () => {
-    server = createServer((req, res) => {
+    server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(200, 'OK', {
         'Content-Length': 2
       })
diff --git a/test/fetch/content-length.js b/test/fetch/content-length.js
index 637ff0b800e..496b4de1b26 100644
--- a/test/fetch/content-length.js
+++ b/test/fetch/content-length.js
@@ -10,7 +10,7 @@ const { closeServerAsPromise } = require('../utils/node-http')
 
 // https://github.com/nodejs/undici/issues/1783
 test('Content-Length is set when using a FormData body with fetch', async (t) => {
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     // TODO: check the length's value once the boundary has a fixed length
     assert.ok('content-length' in req.headers) // request has content-length header
     assert.ok(!Number.isNaN(Number(req.headers['content-length'])))
diff --git a/test/fetch/cookies.js b/test/fetch/cookies.js
index 3bc69c6837b..ac30938dbad 100644
--- a/test/fetch/cookies.js
+++ b/test/fetch/cookies.js
@@ -12,7 +12,7 @@ const { createSecureServer } = require('node:http2')
 const { closeClientAndServerAsPromise } = require('../utils/node-http')
 
 test('Can receive set-cookie headers from a server using fetch - issue #1262', async (t) => {
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('set-cookie', 'name=value; Domain=example.com')
     res.end()
   }).listen(0)
@@ -32,7 +32,7 @@ test('Can receive set-cookie headers from a server using fetch - issue #1262', a
 })
 
 test('Can send cookies to a server with fetch - issue #1463', async (t) => {
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     assert.strictEqual(req.headers.cookie, 'value')
     res.end()
   }).listen(0)
@@ -54,7 +54,7 @@ test('Can send cookies to a server with fetch - issue #1463', async (t) => {
 test('Cookie header is delimited with a semicolon rather than a comma - issue #1905', async (t) => {
   const { strictEqual } = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     strictEqual(req.headers.cookie, 'FOO=lorem-ipsum-dolor-sit-amet; BAR=the-quick-brown-fox')
     res.end()
   }).listen(0)
diff --git a/test/fetch/encoding.js b/test/fetch/encoding.js
index 93c98274b08..9941fc68885 100644
--- a/test/fetch/encoding.js
+++ b/test/fetch/encoding.js
@@ -12,7 +12,7 @@ test('content-encoding header is case-iNsENsITIve', async (t) => {
   const contentCodings = 'GZiP, bR'
   const text = 'Hello, World!'
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     const gzip = createGzip()
     const brotli = createBrotliCompress()
 
@@ -38,7 +38,7 @@ test('response decompression according to content-encoding should be handled in
   const contentCodings = 'deflate, gzip'
   const text = 'Hello, World!'
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     const gzip = createGzip()
     const deflate = createDeflate()
 
diff --git a/test/fetch/exiting.js b/test/fetch/exiting.js
index 0fb007be513..746a7fd97ee 100644
--- a/test/fetch/exiting.js
+++ b/test/fetch/exiting.js
@@ -8,7 +8,7 @@ const tspl = require('@matteo.collina/tspl')
 
 test('abort the request on the other side if the stream is canceled', async (t) => {
   const p = tspl(t, { plan: 1 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200)
     res.write('hello')
     req.on('aborted', () => {
diff --git a/test/fetch/fetch-leak.js b/test/fetch/fetch-leak.js
index caf75fe357b..761a63f9700 100644
--- a/test/fetch/fetch-leak.js
+++ b/test/fetch/fetch-leak.js
@@ -14,7 +14,7 @@ test('do not leak', (t, done) => {
     throw new Error('gc is not available. Run with \'--expose-gc\'.')
   }
   const { ok } = tspl(t, { plan: 1 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   t.after(closeServerAsPromise(server))
diff --git a/test/fetch/fetch-timeouts.js b/test/fetch/fetch-timeouts.js
index 038c23b5af9..41f9484e01d 100644
--- a/test/fetch/fetch-timeouts.js
+++ b/test/fetch/fetch-timeouts.js
@@ -24,7 +24,7 @@ test('Fetch very long request, timeout overridden so no error', (t, done) => {
     Object.assign(timers, orgTimers)
   })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       res.end('hello')
     }, msToDelay)
diff --git a/test/fetch/fetch-url-after-redirect.js b/test/fetch/fetch-url-after-redirect.js
index e387848f8e4..c24c092e15b 100644
--- a/test/fetch/fetch-url-after-redirect.js
+++ b/test/fetch/fetch-url-after-redirect.js
@@ -9,7 +9,7 @@ const { promisify } = require('node:util')
 
 test('after redirecting the url of the response is set to the target url', async (t) => {
   // redirect-1 -> redirect-2 -> target
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     switch (res.req.url) {
       case '/redirect-1':
         res.writeHead(302, undefined, { Location: '/redirect-2' })
@@ -41,7 +41,7 @@ test('after redirecting the url of the response is set to the target url', async
 
 test('location header with non-ASCII character redirects to a properly encoded url', async (t) => {
   // redirect -> %EC%95%88%EB%85%95 (안녕), not %C3%AC%C2%95%C2%88%C3%AB%C2%85%C2%95
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (res.req.url.endsWith('/redirect')) {
       res.writeHead(302, undefined, { Location: `/${Buffer.from('안녕').toString('binary')}` })
       res.end()
diff --git a/test/fetch/fire-and-forget.js b/test/fetch/fire-and-forget.js
index 5794f738cd2..40bb636e468 100644
--- a/test/fetch/fire-and-forget.js
+++ b/test/fetch/fire-and-forget.js
@@ -22,7 +22,7 @@ test('does not need the body to be consumed to continue', { timeout: 180_000, sk
     keepAliveTimeoutThreshold: 10
   })
   setGlobalDispatcher(agent)
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200)
     res.end(blob)
   })
diff --git a/test/fetch/headers-case.js b/test/fetch/headers-case.js
index 78c10b3d547..e0314565bf7 100644
--- a/test/fetch/headers-case.js
+++ b/test/fetch/headers-case.js
@@ -10,7 +10,7 @@ const { closeServerAsPromise } = require('../utils/node-http')
 test('Headers retain keys case-sensitive', async (t) => {
   const assert = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     assert.ok(req.rawHeaders.includes('Content-Type'))
 
     res.end()
diff --git a/test/fetch/headers.js b/test/fetch/headers.js
index 02d500d6f80..1430abff1a9 100644
--- a/test/fetch/headers.js
+++ b/test/fetch/headers.js
@@ -694,7 +694,7 @@ test('Headers.prototype.getSetCookie', async (t) => {
 
   // https://github.com/nodejs/undici/issues/1935
   await t.test('When Headers are cloned, so are the cookies (single entry)', async (t) => {
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.setHeader('Set-Cookie', 'test=onetwo')
       res.end('Hello World!')
     }).listen(0)
@@ -710,7 +710,7 @@ test('Headers.prototype.getSetCookie', async (t) => {
   })
 
   await t.test('When Headers are cloned, so are the cookies (multiple entries)', async (t) => {
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.setHeader('Set-Cookie', ['test=onetwo', 'test=onetwothree'])
       res.end('Hello World!')
     }).listen(0)
diff --git a/test/fetch/integrity.js b/test/fetch/integrity.js
index b88780175f8..5c34a87e20f 100644
--- a/test/fetch/integrity.js
+++ b/test/fetch/integrity.js
@@ -21,7 +21,7 @@ test('request with correct integrity checksum', (t, done) => {
   const body = 'Hello world!'
   const hash = createHash('sha256').update(body).digest('base64')
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(body)
   })
 
@@ -40,7 +40,7 @@ test('request with wrong integrity checksum', async (t) => {
   const body = 'Hello world!'
   const hash = 'c0535e4be2b79ffd93291305436bf889314e4a3faec05ecffcbb7df31ad9e51b'
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(body)
   }).listen(0)
 
@@ -60,7 +60,7 @@ test('request with integrity checksum on encoded body', (t, done) => {
   const body = 'Hello world!'
   const hash = createHash('sha256').update(body).digest('base64')
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-encoding', 'gzip')
     res.end(gzipSync(body))
   })
@@ -77,7 +77,7 @@ test('request with integrity checksum on encoded body', (t, done) => {
 })
 
 test('request with a totally incorrect integrity', async (t) => {
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   }).listen(0)
 
@@ -93,7 +93,7 @@ test('request with mixed in/valid integrities', async (t) => {
   const body = 'Hello world!'
   const hash = createHash('sha256').update(body).digest('base64')
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(body)
   }).listen(0)
 
@@ -109,7 +109,7 @@ test('request with sha384 hash', { skip: !supportedHashes.includes('sha384') },
   const body = 'Hello world!'
   const hash = createHash('sha384').update(body).digest('base64')
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(body)
   }).listen(0)
 
@@ -131,7 +131,7 @@ test('request with sha512 hash', { skip: !supportedHashes.includes('sha512') },
   const body = 'Hello world!'
   const hash = createHash('sha512').update(body).digest('base64')
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(body)
   }).listen(0)
 
@@ -154,7 +154,7 @@ test('request with correct integrity checksum (base64url)', async (t) => {
   const body = 'Hello world!'
   const hash = createHash('sha256').update(body).digest('base64url')
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(body)
   })
 
@@ -176,7 +176,7 @@ test('request with incorrect integrity checksum (base64url)', async (t) => {
   const body = 'Hello world!'
   const hash = createHash('sha256').update('invalid').digest('base64url')
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(body)
   })
 
@@ -196,7 +196,7 @@ test('request with incorrect integrity checksum (only dash)', async (t) => {
 
   const body = 'Hello world!'
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(body)
   })
 
@@ -216,7 +216,7 @@ test('request with incorrect integrity checksum (non-ascii character)', async (t
 
   const body = 'Hello world!'
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(body)
   })
 
@@ -238,7 +238,7 @@ test('request with incorrect stronger integrity checksum (non-ascii character)',
   const sha256 = createHash('sha256').update(body).digest('base64')
   const sha384 = 'ä'
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(body)
   })
 
@@ -263,7 +263,7 @@ test('request with correct integrity checksum (base64). mixed', async (t) => {
   const sha384 = createHash('sha384').update(body).digest('base64')
   const sha512 = createHash('sha512').update(body).digest('base64')
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(body)
   })
 
@@ -309,7 +309,7 @@ test('request with correct integrity checksum (base64url). mixed', async (t) =>
   const sha384 = createHash('sha384').update(body).digest('base64url')
   const sha512 = createHash('sha512').update(body).digest('base64url')
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(body)
   })
 
diff --git a/test/fetch/issue-1711.js b/test/fetch/issue-1711.js
index be48d160c20..ca78897ae5f 100644
--- a/test/fetch/issue-1711.js
+++ b/test/fetch/issue-1711.js
@@ -9,7 +9,7 @@ const { fetch } = require('../..')
 test('Redirecting a bunch does not cause a MaxListenersExceededWarning', async (t) => {
   let redirects = 0
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (redirects === 15) {
       res.end('Okay goodbye')
       return
@@ -36,7 +36,7 @@ test(
   'aborting a Stream throws',
   () => {
     return new Promise((resolve, reject) => {
-      const httpServer = createServer((request, response) => {
+      const httpServer = createServer({ joinDuplicateHeaders: true }, (request, response) => {
         response.end(new Uint8Array(20000))
       }).listen(async () => {
         const serverAddress = httpServer.address()
diff --git a/test/fetch/issue-2009.js b/test/fetch/issue-2009.js
index d77b60b6334..14432e81cb2 100644
--- a/test/fetch/issue-2009.js
+++ b/test/fetch/issue-2009.js
@@ -10,7 +10,7 @@ const { closeServerAsPromise } = require('../utils/node-http')
 test('issue 2009', async (t) => {
   const { doesNotReject } = tspl(t, { plan: 10 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('a', 'b')
     res.flushHeaders()
 
diff --git a/test/fetch/issue-2021.js b/test/fetch/issue-2021.js
index 5b949e507de..b88db725dbe 100644
--- a/test/fetch/issue-2021.js
+++ b/test/fetch/issue-2021.js
@@ -9,7 +9,7 @@ const { closeServerAsPromise } = require('../utils/node-http')
 
 // https://github.com/nodejs/undici/issues/2021
 test('content-length header is removed on redirect', async (t) => {
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (req.url === '/redirect') {
       res.writeHead(302, { Location: '/redirect2' })
       res.end()
diff --git a/test/fetch/issue-2171.js b/test/fetch/issue-2171.js
index 82577f9494c..ba0c82674cf 100644
--- a/test/fetch/issue-2171.js
+++ b/test/fetch/issue-2171.js
@@ -8,7 +8,7 @@ const assert = require('node:assert')
 const { closeServerAsPromise } = require('../utils/node-http')
 
 test('error reason is forwarded - issue #2171', { skip: !AbortSignal.timeout }, async (t) => {
-  const server = createServer(() => {}).listen(0)
+  const server = createServer({ joinDuplicateHeaders: true }, () => {}).listen(0)
 
   t.after(closeServerAsPromise(server))
   await once(server, 'listening')
diff --git a/test/fetch/issue-2318.js b/test/fetch/issue-2318.js
index 273e2b9a74c..fe27ac7ec35 100644
--- a/test/fetch/issue-2318.js
+++ b/test/fetch/issue-2318.js
@@ -10,7 +10,7 @@ const { closeServerAsPromise } = require('../utils/node-http')
 test('Undici overrides user-provided `Host` header', async (t) => {
   const { strictEqual } = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     strictEqual(req.headers.host, `localhost:${server.address().port}`)
 
     res.end()
diff --git a/test/fetch/issue-2828.js b/test/fetch/issue-2828.js
index 08e8ea714ca..98fd375b8e9 100644
--- a/test/fetch/issue-2828.js
+++ b/test/fetch/issue-2828.js
@@ -16,7 +16,7 @@ test('issue #2828, dispatcher is allowed in RequestInit options', async (t) => {
     }
   }
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     deepStrictEqual(req.headers['x-my-header'], 'hello')
     res.end()
   }).listen(0)
diff --git a/test/fetch/issue-2898-comment.js b/test/fetch/issue-2898-comment.js
index c46b80b3ff7..d1d326b2c35 100644
--- a/test/fetch/issue-2898-comment.js
+++ b/test/fetch/issue-2898-comment.js
@@ -23,7 +23,7 @@ test('issue #2828, RequestInit dispatcher options overrides Request input dispat
     }
   }
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     strictEqual(req.headers['x-my-header-a'], undefined)
     strictEqual(req.headers['x-my-header-b'], 'world')
     res.end()
diff --git a/test/fetch/issue-2898.js b/test/fetch/issue-2898.js
index 231b7615761..d6188ab696a 100644
--- a/test/fetch/issue-2898.js
+++ b/test/fetch/issue-2898.js
@@ -10,7 +10,7 @@ const { fetch } = require('../..')
 test('421 requests with a body work as expected', async (t) => {
   const expected = 'This is a 421 Misdirected Request response.'
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.statusCode = 421
     res.end(expected)
   }).listen(0)
diff --git a/test/fetch/issue-3334.js b/test/fetch/issue-3334.js
index 968b2a797b7..1fcfd0e86c7 100644
--- a/test/fetch/issue-3334.js
+++ b/test/fetch/issue-3334.js
@@ -10,7 +10,7 @@ test('a non-empty origin is not appended (issue #3334)', async (t) => {
   const { strictEqual } = tspl(t, { plan: 1 })
   const origin = 'https://origin.example.com'
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     strictEqual(req.headers.origin, origin)
     res.end()
   }).listen(0)
diff --git a/test/fetch/issue-3616.js b/test/fetch/issue-3616.js
index ed9f739bba1..2de5c63672d 100644
--- a/test/fetch/issue-3616.js
+++ b/test/fetch/issue-3616.js
@@ -17,7 +17,7 @@ describe('https://github.com/nodejs/undici/issues/3616', () => {
   for (const encoding of cases) {
     test(encoding, async t => {
       t = tspl(t, { plan: 2 })
-      const server = createServer((req, res) => {
+      const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
         res.writeHead(200, {
           'Content-Length': '0',
           Connection: 'close',
diff --git a/test/fetch/issue-3624.js b/test/fetch/issue-3624.js
index 37e722d7e4e..487fed7b210 100644
--- a/test/fetch/issue-3624.js
+++ b/test/fetch/issue-3624.js
@@ -9,7 +9,7 @@ const { fetch, FormData } = require('../..')
 
 // https://github.com/nodejs/undici/issues/3624
 test('crlf is appended to formdata body (issue #3624)', async (t) => {
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.pipe(res)
   }).listen(0)
 
diff --git a/test/fetch/issue-3767.js b/test/fetch/issue-3767.js
index fec7c71dbe6..aa742aaffe6 100644
--- a/test/fetch/issue-3767.js
+++ b/test/fetch/issue-3767.js
@@ -12,7 +12,7 @@ test('referrerPolicy unsafe-url is respected', async (t) => {
 
   const referrer = 'https://google.com/hello/world'
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     deepEqual(req.headers.referer, referrer)
 
     res.end()
diff --git a/test/fetch/issue-node-46525.js b/test/fetch/issue-node-46525.js
index b35eeb2a590..da7c2cd27b6 100644
--- a/test/fetch/issue-node-46525.js
+++ b/test/fetch/issue-node-46525.js
@@ -11,7 +11,7 @@ test('No warning when reusing AbortController', async (t) => {
     throw new Error('Got warning')
   }
 
-  const server = createServer((req, res) => res.end()).listen(0)
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => res.end()).listen(0)
 
   await once(server, 'listening')
 
diff --git a/test/fetch/issue-rsshub-15532.js b/test/fetch/issue-rsshub-15532.js
index 11491337de5..988e12ceb5c 100644
--- a/test/fetch/issue-rsshub-15532.js
+++ b/test/fetch/issue-rsshub-15532.js
@@ -10,7 +10,7 @@ const { tspl } = require('@matteo.collina/tspl')
 test('An invalid Origin header is not set', async (t) => {
   const { deepStrictEqual } = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     deepStrictEqual(req.headers.origin, undefined)
 
     res.end()
diff --git a/test/fetch/long-lived-abort-controller.js b/test/fetch/long-lived-abort-controller.js
index 561ebee0661..427b605e295 100644
--- a/test/fetch/long-lived-abort-controller.js
+++ b/test/fetch/long-lived-abort-controller.js
@@ -10,7 +10,7 @@ const { strictEqual } = require('node:assert')
 // const isNode18 = process.version.startsWith('v18')
 
 test('long-lived-abort-controller', { skip: true }, async (t) => {
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200, { 'Content-Type': 'text/plain' })
     res.write('Hello World!')
     res.end()
diff --git a/test/fetch/pull-dont-push.js b/test/fetch/pull-dont-push.js
index 27454bd6c58..7489b3fb0cd 100644
--- a/test/fetch/pull-dont-push.js
+++ b/test/fetch/pull-dont-push.js
@@ -14,7 +14,7 @@ test('pull dont\'t push', async (t) => {
   let count = 0
   let socket
   const max = 1_000_000
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.statusCode = 200
     socket = res.socket
 
diff --git a/test/fetch/redirect-cross-origin-header.js b/test/fetch/redirect-cross-origin-header.js
index 3756c22d417..8f0d58587e7 100644
--- a/test/fetch/redirect-cross-origin-header.js
+++ b/test/fetch/redirect-cross-origin-header.js
@@ -9,7 +9,7 @@ const { fetch } = require('../..')
 test('Cross-origin redirects clear forbidden headers', async (t) => {
   const { strictEqual } = tspl(t, { plan: 6 })
 
-  const server1 = createServer((req, res) => {
+  const server1 = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     strictEqual(req.headers.cookie, undefined)
     strictEqual(req.headers.authorization, undefined)
     strictEqual(req.headers['proxy-authorization'], undefined)
@@ -17,7 +17,7 @@ test('Cross-origin redirects clear forbidden headers', async (t) => {
     res.end('redirected')
   }).listen(0)
 
-  const server2 = createServer((req, res) => {
+  const server2 = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     strictEqual(req.headers.authorization, 'test')
     strictEqual(req.headers.cookie, 'ddd=dddd')
 
diff --git a/test/fetch/redirect.js b/test/fetch/redirect.js
index a9533bcc327..6d41ff30849 100644
--- a/test/fetch/redirect.js
+++ b/test/fetch/redirect.js
@@ -9,7 +9,7 @@ const { closeServerAsPromise } = require('../utils/node-http')
 
 // https://github.com/nodejs/undici/issues/1776
 test('Redirecting with a body does not cancel the current request - #1776', async (t) => {
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (req.url === '/redirect') {
       res.statusCode = 301
       res.setHeader('location', '/redirect/')
@@ -31,7 +31,7 @@ test('Redirecting with a body does not cancel the current request - #1776', asyn
 })
 
 test('Redirecting with an empty body does not throw an error - #2027', async (t) => {
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (req.url === '/redirect') {
       res.statusCode = 307
       res.setHeader('location', '/redirect/')
@@ -52,7 +52,7 @@ test('Redirecting with an empty body does not throw an error - #2027', async (t)
 })
 
 test('Redirecting with a body does not fail to write body - #2543', async (t) => {
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (req.url === '/redirect') {
       res.writeHead(307, { location: '/target' })
       res.write('<a href="/redirect/">Moved Permanently</a>')
diff --git a/test/fetch/referrrer-policy.js b/test/fetch/referrrer-policy.js
index e2e8ee90002..612d8e29d0d 100644
--- a/test/fetch/referrrer-policy.js
+++ b/test/fetch/referrrer-policy.js
@@ -73,7 +73,7 @@ describe('referrer-policy', () => {
     test(title, async (t) => {
       t = tspl(t, { plan: 1 })
 
-      const server = createServer((req, res) => {
+      const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
         switch (res.req.url) {
           case '/redirect':
             res.writeHead(302, undefined, {
diff --git a/test/fetch/relative-url.js b/test/fetch/relative-url.js
index 2a57e15c5e5..b6e28bda7ea 100644
--- a/test/fetch/relative-url.js
+++ b/test/fetch/relative-url.js
@@ -75,7 +75,7 @@ test('fetch', async (t) => {
   await assert.rejects(fetch('/relative/path'), TypeError('Failed to parse URL from /relative/path'))
 
   await t.test('Basic fetch', async (t) => {
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       assert.strictEqual(req.url, '/relative/path')
       res.end()
     }).listen(0)
@@ -88,7 +88,7 @@ test('fetch', async (t) => {
   })
 
   await t.test('fetch return', async (t) => {
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       assert.strictEqual(req.url, '/relative/path')
       res.end()
     }).listen(0)
diff --git a/test/fetch/resource-timing.js b/test/fetch/resource-timing.js
index b32fd08fb8b..4def7bf90e2 100644
--- a/test/fetch/resource-timing.js
+++ b/test/fetch/resource-timing.js
@@ -37,7 +37,7 @@ test('should create a PerformanceResourceTiming after each fetch request', (t, d
 
   obs.observe({ entryTypes: ['resource'] })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ok')
   }).listen(0, async () => {
     const body = await fetch(`http://localhost:${server.address().port}`)
@@ -62,7 +62,7 @@ test('should include encodedBodySize in performance entry', (t, done) => {
 
   obs.observe({ entryTypes: ['resource'] })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ok')
   }).listen(0, async () => {
     const body = await fetch(`http://localhost:${server.address().port}`)
@@ -98,7 +98,7 @@ test('timing entries should be in order', (t, done) => {
 
   obs.observe({ entryTypes: ['resource'] })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ok')
   }).listen(0, async () => {
     const body = await fetch(`http://localhost:${server.address().port}/redirect`)
@@ -124,7 +124,7 @@ test('redirect timing entries should be included when redirecting', (t, done) =>
 
   obs.observe({ entryTypes: ['resource'] })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (req.url === '/redirect') {
       res.statusCode = 307
       res.setHeader('location', '/redirect/')
diff --git a/test/fetch/user-agent.js b/test/fetch/user-agent.js
index da8936c8cff..770f550725f 100644
--- a/test/fetch/user-agent.js
+++ b/test/fetch/user-agent.js
@@ -10,7 +10,7 @@ const { closeServerAsPromise } = require('../utils/node-http')
 const nodeBuild = require('../../undici-fetch.js')
 
 test('user-agent defaults correctly', async (t) => {
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(JSON.stringify({ userAgentHeader: req.headers['user-agent'] }))
   })
   t.after(closeServerAsPromise(server))
diff --git a/test/fixtures/cache-tests/test-engine/server/server.mjs b/test/fixtures/cache-tests/test-engine/server/server.mjs
index 58f098ba77f..54f644019c7 100644
--- a/test/fixtures/cache-tests/test-engine/server/server.mjs
+++ b/test/fixtures/cache-tests/test-engine/server/server.mjs
@@ -39,11 +39,12 @@ let server
 if (protocol.toLowerCase() === 'https') {
   const options = {
     key: fs.readFileSync(process.env.npm_config_keyfile),
-    cert: fs.readFileSync(process.env.npm_config_certfile)
+    cert: fs.readFileSync(process.env.npm_config_certfile),
+    joinDuplicateHeaders: true
   }
   server = https.createServer(options, handleMain)
 } else {
-  server = http.createServer(handleMain)
+  server = http.createServer({ joinDuplicateHeaders: true }, handleMain)
 }
 server.on('listening', () => {
   const host = (server.address().family === 'IPv6')
diff --git a/test/fixtures/fetch.js b/test/fixtures/fetch.js
index 36e572cd3d1..7d5790fa418 100644
--- a/test/fixtures/fetch.js
+++ b/test/fixtures/fetch.js
@@ -3,7 +3,7 @@
 const { createServer } = require('node:http')
 const { fetch } = require('../..')
 
-const server = createServer((_req, res) => {
+const server = createServer({ joinDuplicateHeaders: true }, (_req, res) => {
   res.writeHead(200, { 'Content-Type': 'text/plain' })
   res.end('hello world')
 })
diff --git a/test/fixtures/interceptors/retry-event-loop.js b/test/fixtures/interceptors/retry-event-loop.js
index 0a92d0d27a6..3fcfa979637 100644
--- a/test/fixtures/interceptors/retry-event-loop.js
+++ b/test/fixtures/interceptors/retry-event-loop.js
@@ -7,7 +7,7 @@ const {
   interceptors: { retry }
 } = require('../../..')
 
-const server = createServer()
+const server = createServer({ joinDuplicateHeaders: true })
 
 server.on('request', (req, res) => {
   res.writeHead(418, { 'Content-Type': 'text/plain' })
diff --git a/test/fixtures/undici.js b/test/fixtures/undici.js
index 643142501ff..6bc6ba0fc3e 100644
--- a/test/fixtures/undici.js
+++ b/test/fixtures/undici.js
@@ -3,7 +3,7 @@
 const { createServer } = require('node:http')
 const { request } = require('../..')
 
-const server = createServer((_req, res) => {
+const server = createServer({ joinDuplicateHeaders: true }, (_req, res) => {
   res.writeHead(200, { 'Content-Type': 'text/plain' })
   res.end('hello world')
 })
diff --git a/test/get-head-body.js b/test/get-head-body.js
index 4e58bc0f5a5..4168cf23090 100644
--- a/test/get-head-body.js
+++ b/test/get-head-body.js
@@ -12,7 +12,7 @@ const { wrapWithAsyncIterable } = require('./utils/async-iterators')
 test('GET and HEAD with body should reset connection', async (t) => {
   t = tspl(t, { plan: 8 + 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
 
@@ -140,7 +140,7 @@ test('GET and HEAD with body should reset connection', async (t) => {
 test('HEAD should reset connection', async (t) => {
   t = tspl(t, { plan: 8 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
 
diff --git a/test/headers-as-array.js b/test/headers-as-array.js
index 693979e92b7..9fd4e738af8 100644
--- a/test/headers-as-array.js
+++ b/test/headers-as-array.js
@@ -9,7 +9,7 @@ test('handle headers as array', async (t) => {
   t = tspl(t, { plan: 3 })
   const headers = ['a', '1', 'b', '2', 'c', '3']
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.headers.a, '1')
     t.strictEqual(req.headers.b, '2')
     t.strictEqual(req.headers.c, '3')
@@ -34,7 +34,7 @@ test('handle multi-valued headers as array', async (t) => {
   t = tspl(t, { plan: 4 })
   const headers = ['a', '1', 'b', '2', 'c', '3', 'd', '4', 'd', '5']
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.headers.a, '1')
     t.strictEqual(req.headers.b, '2')
     t.strictEqual(req.headers.c, '3')
@@ -60,7 +60,7 @@ test('handle headers with array', async (t) => {
   t = tspl(t, { plan: 4 })
   const headers = { a: '1', b: '2', c: '3', d: ['4'] }
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.headers.a, '1')
     t.strictEqual(req.headers.b, '2')
     t.strictEqual(req.headers.c, '3')
@@ -86,7 +86,7 @@ test('handle multi-valued headers', async (t) => {
   t = tspl(t, { plan: 4 })
   const headers = { a: '1', b: '2', c: '3', d: ['4', '5'] }
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.headers.a, '1')
     t.strictEqual(req.headers.b, '2')
     t.strictEqual(req.headers.c, '3')
@@ -112,7 +112,7 @@ test('fail if headers array is odd', async (t) => {
   t = tspl(t, { plan: 2 })
   const headers = ['a', '1', 'b', '2', 'c', '3', 'd']
 
-  const server = createServer((req, res) => { res.end() })
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => { res.end() })
   after(() => server.close())
   server.listen(0, () => {
     const client = new Client(`http://localhost:${server.address().port}`)
@@ -135,7 +135,7 @@ test('fail if headers is not an object or an array', async (t) => {
   t = tspl(t, { plan: 2 })
   const headers = 'not an object or an array'
 
-  const server = createServer((req, res) => { res.end() })
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => { res.end() })
   after(() => server.close())
   server.listen(0, () => {
     const client = new Client(`http://localhost:${server.address().port}`)
diff --git a/test/headers-crlf.js b/test/headers-crlf.js
index 8660259dbbd..ee440e06045 100644
--- a/test/headers-crlf.js
+++ b/test/headers-crlf.js
@@ -9,7 +9,7 @@ const { createServer } = require('node:http')
 test('CRLF Injection in Nodejs ‘undici’ via host', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     res.end()
   })
   after(() => server.close())
diff --git a/test/http-100.js b/test/http-100.js
index d961a2c421b..055a01faaa5 100644
--- a/test/http-100.js
+++ b/test/http-100.js
@@ -10,7 +10,7 @@ const { once } = require('node:events')
 test('ignore informational response', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeProcessing()
     req.pipe(res)
   })
@@ -42,7 +42,7 @@ test('ignore informational response', async (t) => {
 test('error 103 body', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = net.createServer((socket) => {
+  const server = net.createServer({ joinDuplicateHeaders: true }, (socket) => {
     socket.write('HTTP/1.1 103 Early Hints\r\n')
     socket.write('Content-Length: 1\r\n')
     socket.write('\r\n')
@@ -71,7 +71,7 @@ test('error 103 body', async (t) => {
 test('error 100 body', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = net.createServer((socket) => {
+  const server = net.createServer({ joinDuplicateHeaders: true }, (socket) => {
     socket.write('HTTP/1.1 100 Early Hints\r\n')
     socket.write('\r\n')
   })
@@ -97,7 +97,7 @@ test('error 100 body', async (t) => {
 test('error 101 upgrade', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = net.createServer((socket) => {
+  const server = net.createServer({ joinDuplicateHeaders: true }, (socket) => {
     socket.write('HTTP/1.1 101 Switching Protocols\r\nUpgrade: example/1\r\nConnection: Upgrade\r\n')
     socket.write('\r\n')
   })
@@ -123,7 +123,7 @@ test('error 101 upgrade', async (t) => {
 test('1xx response without timeouts', async t => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeProcessing()
     setTimeout(() => req.pipe(res), 2000)
   })
diff --git a/test/http-req-destroy.js b/test/http-req-destroy.js
index ea7624c611d..6c748b950f6 100644
--- a/test/http-req-destroy.js
+++ b/test/http-req-destroy.js
@@ -11,7 +11,7 @@ function doNotKillReqSocket (bodyType) {
   test(`do not kill req socket ${bodyType}`, async (t) => {
     t = tspl(t, { plan: 3 })
 
-    const server1 = createServer((req, res) => {
+    const server1 = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       const client = new undici.Client(`http://localhost:${server2.address().port}`)
       after(() => client.close())
       client.request({
@@ -32,7 +32,7 @@ function doNotKillReqSocket (bodyType) {
     })
     after(() => server1.close())
 
-    const server2 = createServer((req, res) => {
+    const server2 = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       setTimeout(() => {
         req.pipe(res)
       }, 100)
diff --git a/test/https.js b/test/https.js
index 418fb969f45..89e55b6a5ae 100644
--- a/test/https.js
+++ b/test/https.js
@@ -9,7 +9,7 @@ const pem = require('https-pem')
 test('https get with tls opts', async (t) => {
   t = tspl(t, { plan: 6 })
 
-  const server = createServer(pem, (req, res) => {
+  const server = createServer({ ...pem, joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     res.setHeader('content-type', 'text/plain')
@@ -45,7 +45,7 @@ test('https get with tls opts', async (t) => {
 test('https get with tls opts ip', async (t) => {
   t = tspl(t, { plan: 6 })
 
-  const server = createServer(pem, (req, res) => {
+  const server = createServer({ ...pem, joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     res.setHeader('content-type', 'text/plain')
diff --git a/test/inflight-and-close.js b/test/inflight-and-close.js
index 075cdba1668..3e0a532bb9c 100644
--- a/test/inflight-and-close.js
+++ b/test/inflight-and-close.js
@@ -8,7 +8,7 @@ const http = require('node:http')
 test('inflight and close', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200)
     res.end('Response body')
     res.socket.end() // Close the connection immediately with every response
diff --git a/test/interceptors/cache-fastimers-fix.js b/test/interceptors/cache-fastimers-fix.js
index 65650ceefc4..6fd45d861c3 100644
--- a/test/interceptors/cache-fastimers-fix.js
+++ b/test/interceptors/cache-fastimers-fix.js
@@ -9,7 +9,7 @@ const { setTimeout: sleep } = require('timers/promises')
 
 test('revalidates the request when the response is stale', async () => {
   let count = 0
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('Cache-Control', 'public, max-age=1')
     res.end('hello world ' + count++)
   })
diff --git a/test/interceptors/cache.js b/test/interceptors/cache.js
index dc120c60b7b..f658a4701f6 100644
--- a/test/interceptors/cache.js
+++ b/test/interceptors/cache.js
@@ -10,7 +10,7 @@ const { Client, interceptors, cacheStores: { MemoryCacheStore } } = require('../
 describe('Cache Interceptor', () => {
   test('caches request', async () => {
     let requestsToOrigin = 0
-    const server = createServer((_, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (_, res) => {
       requestsToOrigin++
       res.setHeader('cache-control', 's-maxage=10')
       res.end('asd')
@@ -53,7 +53,7 @@ describe('Cache Interceptor', () => {
 
   test('vary directives used to decide which response to use', async () => {
     let requestsToOrigin = 0
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       requestsToOrigin++
       res.setHeader('cache-control', 's-maxage=10')
       res.setHeader('vary', 'a')
@@ -135,7 +135,7 @@ describe('Cache Interceptor', () => {
 
     let requestsToOrigin = 0
     let revalidationRequests = 0
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.setHeader('date', 0)
       res.setHeader('cache-control', 's-maxage=1, stale-while-revalidate=10')
 
@@ -220,7 +220,7 @@ describe('Cache Interceptor', () => {
     let requestsToOrigin = 0
     let revalidationRequests = 0
     let serverError
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.setHeader('date', 0)
       res.setHeader('cache-control', 's-maxage=1, stale-while-revalidate=10')
 
@@ -317,7 +317,7 @@ describe('Cache Interceptor', () => {
     let requestsToOrigin = 0
     let revalidationRequests = 0
     let serverError
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.setHeader('date', 0)
       res.setHeader('cache-control', 's-maxage=1, stale-while-revalidate=10')
 
@@ -392,7 +392,7 @@ describe('Cache Interceptor', () => {
   })
 
   test('unsafe methods cause resource to be purged from cache', async () => {
-    const server = createServer((_, res) => res.end('asd')).listen(0)
+    const server = createServer({ joinDuplicateHeaders: true }, (_, res) => res.end('asd')).listen(0)
 
     after(() => server.close())
     await once(server, 'listening')
@@ -441,7 +441,7 @@ describe('Cache Interceptor', () => {
   })
 
   test('unsafe methods aren\'t cached', async () => {
-    const server = createServer((_, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (_, res) => {
       res.setHeader('cache-control', 'public, s-maxage=1')
       res.end('')
     }).listen(0)
@@ -485,7 +485,7 @@ describe('Cache Interceptor', () => {
     ]
 
     let requestToOrigin = 0
-    const server = createServer((_, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (_, res) => {
       requestToOrigin++
       res.setHeader('cache-control', 's-maxage=10, no-cache=should-be-stripped')
       res.setHeader('should-not-be-stripped', 'asd')
@@ -540,7 +540,7 @@ describe('Cache Interceptor', () => {
 
   test('cacheByDefault', async () => {
     let requestsToOrigin = 0
-    const server = createServer((_, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (_, res) => {
       requestsToOrigin++
       res.end('asd')
     }).listen(0)
@@ -583,7 +583,7 @@ describe('Cache Interceptor', () => {
     })
 
     let requestsToOrigin = 0
-    const server = createServer((_, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (_, res) => {
       res.setHeader('date', 0)
 
       requestsToOrigin++
@@ -665,7 +665,7 @@ describe('Cache Interceptor', () => {
       })
 
       let requestsToOrigin = 0
-      const server = createServer((_, res) => {
+      const server = createServer({ joinDuplicateHeaders: true }, (_, res) => {
         requestsToOrigin++
         res.setHeader('date', 0)
         res.setHeader('cache-control', 'public, s-maxage=100')
@@ -727,7 +727,7 @@ describe('Cache Interceptor', () => {
 
       let requestsToOrigin = 0
       let revalidationRequests = 0
-      const server = createServer((req, res) => {
+      const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
         res.setHeader('date', 0)
         res.setHeader('cache-control', 'public, s-maxage=1, stale-while-revalidate=10')
 
@@ -796,7 +796,7 @@ describe('Cache Interceptor', () => {
       })
 
       let requestsToOrigin = 0
-      const server = createServer((_, res) => {
+      const server = createServer({ joinDuplicateHeaders: true }, (_, res) => {
         requestsToOrigin++
         res.setHeader('date', 0)
         res.setHeader('cache-control', 'public, s-maxage=10')
@@ -855,7 +855,7 @@ describe('Cache Interceptor', () => {
     test('no-cache', async () => {
       let requestsToOrigin = 0
       let revalidationRequests = 0
-      const server = createServer((req, res) => {
+      const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
         if (req.headers['if-modified-since']) {
           revalidationRequests++
           res.statusCode = 304
@@ -914,7 +914,7 @@ describe('Cache Interceptor', () => {
     })
 
     test('no-store', async () => {
-      const server = createServer((_, res) => {
+      const server = createServer({ joinDuplicateHeaders: true }, (_, res) => {
         res.setHeader('cache-control', 'public, s-maxage=100')
         res.end('asd')
       }).listen(0)
@@ -946,7 +946,7 @@ describe('Cache Interceptor', () => {
 
     test('only-if-cached', async () => {
       let requestsToOrigin = 0
-      const server = createServer((_, res) => {
+      const server = createServer({ joinDuplicateHeaders: true }, (_, res) => {
         res.setHeader('cache-control', 'public, s-maxage=100')
         res.end('asd')
         requestsToOrigin++
@@ -1014,7 +1014,7 @@ describe('Cache Interceptor', () => {
       })
 
       let requestsToOrigin = 0
-      const server = createServer((_, res) => {
+      const server = createServer({ joinDuplicateHeaders: true }, (_, res) => {
         res.setHeader('date', 0)
 
         requestsToOrigin++
diff --git a/test/interceptors/dns.js b/test/interceptors/dns.js
index 75fc21faec6..0d7e633bff6 100644
--- a/test/interceptors/dns.js
+++ b/test/interceptors/dns.js
@@ -33,7 +33,7 @@ test('Should automatically resolve IPs (dual stack)', async t => {
   t = tspl(t, { plan: 8 })
 
   const hostsnames = []
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -201,7 +201,7 @@ test('Should recover on network errors (dual stack - 4)', async t => {
   t = tspl(t, { plan: 7 })
 
   let counter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -290,7 +290,7 @@ test('Should recover on network errors (dual stack - 6)', async t => {
   t = tspl(t, { plan: 7 })
 
   let counter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -466,7 +466,7 @@ test('Should automatically resolve IPs (dual stack disabled - 4)', async t => {
   t = tspl(t, { plan: 6 })
 
   let counter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -537,7 +537,7 @@ test('Should automatically resolve IPs (dual stack disabled - 6)', async t => {
   t = tspl(t, { plan: 6 })
 
   let counter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -611,7 +611,7 @@ test('Should we handle TTL (4)', async t => {
   const clock = FakeTimers.install()
   let counter = 0
   let lookupCounter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -714,7 +714,7 @@ test('Should we handle TTL (6)', async t => {
   const clock = FakeTimers.install()
   let counter = 0
   let lookupCounter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -818,7 +818,7 @@ test('Should set lowest TTL between resolved and option maxTTL', async t => {
 
   const clock = FakeTimers.install()
   let lookupCounter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -911,7 +911,7 @@ test('Should use all dns entries (dual stack)', async t => {
 
   let counter = 0
   let lookupCounter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -1001,7 +1001,7 @@ test('Should use all dns entries (dual stack disabled - 4)', async t => {
 
   let counter = 0
   let lookupCounter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -1097,7 +1097,7 @@ test('Should use all dns entries (dual stack disabled - 6)', async t => {
 
   let counter = 0
   let lookupCounter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -1195,7 +1195,7 @@ test('Should handle single family resolved (dual stack)', async t => {
   const clock = FakeTimers.install()
   let counter = 0
   let lookupCounter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -1286,7 +1286,7 @@ test('Should prefer affinity (dual stack - 4)', async t => {
   const clock = FakeTimers.install()
   let counter = 0
   let lookupCounter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -1388,7 +1388,7 @@ test('Should prefer affinity (dual stack - 6)', async t => {
   const clock = FakeTimers.install()
   let counter = 0
   let lookupCounter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -1488,8 +1488,8 @@ test('Should use resolved ports (4)', async t => {
   t = tspl(t, { plan: 5 })
 
   let lookupCounter = 0
-  const server1 = createServer()
-  const server2 = createServer()
+  const server1 = createServer({ joinDuplicateHeaders: true })
+  const server2 = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -1556,8 +1556,8 @@ test('Should use resolved ports (6)', async t => {
   t = tspl(t, { plan: 5 })
 
   let lookupCounter = 0
-  const server1 = createServer()
-  const server2 = createServer()
+  const server1 = createServer({ joinDuplicateHeaders: true })
+  const server2 = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -1624,8 +1624,8 @@ test('Should handle max cached items', async t => {
   t = tspl(t, { plan: 9 })
 
   let counter = 0
-  const server1 = createServer()
-  const server2 = createServer()
+  const server1 = createServer({ joinDuplicateHeaders: true })
+  const server2 = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -1833,7 +1833,7 @@ test('#3937 - Handle host correctly', async t => {
   t = tspl(t, { plan: 10 })
 
   const hostsnames = []
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
diff --git a/test/interceptors/dump-interceptor.js b/test/interceptors/dump-interceptor.js
index ba6dbe83c02..cc29f4d30ba 100644
--- a/test/interceptors/dump-interceptor.js
+++ b/test/interceptors/dump-interceptor.js
@@ -17,7 +17,7 @@ if (platform() === 'win32') {
 test('Should dump on abort', async t => {
   t = tspl(t, { plan: 2 })
   let offset = 0
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     const max = 1024 * 1024
     const buffer = Buffer.alloc(max)
 
@@ -79,7 +79,7 @@ test('Should dump on abort', async t => {
 test('Should dump on already aborted request', async t => {
   t = tspl(t, { plan: 3 })
   let offset = 0
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     const max = 1024
     const buffer = Buffer.alloc(max)
 
@@ -139,7 +139,7 @@ test('Should dump on already aborted request', async t => {
 
 test('Should dump response body up to limit (default)', async t => {
   t = tspl(t, { plan: 3 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     const buffer = Buffer.alloc(1024 * 1024)
     res.writeHead(200, {
       'Content-Length': buffer.length,
@@ -181,7 +181,7 @@ test('Should dump response body up to limit (default)', async t => {
 
 test('Should dump response body up to limit and ignore trailers', async t => {
   t = tspl(t, { plan: 3 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200, {
       'Content-Type': 'text/plain',
       'Transfer-Encoding': 'chunked',
@@ -225,7 +225,7 @@ test('Should dump response body up to limit and ignore trailers', async t => {
 
 test('Should forward common error', async t => {
   t = tspl(t, { plan: 1 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.destroy()
   })
 
@@ -358,7 +358,7 @@ test('Should throw on bad opts', async t => {
 
 test('Should dump response body up to limit (opts)', async t => {
   t = tspl(t, { plan: 3 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     const buffer = Buffer.alloc(1 * 1024)
     res.writeHead(200, {
       'Content-Length': buffer.length,
@@ -399,7 +399,7 @@ test('Should dump response body up to limit (opts)', async t => {
 
 test('Should abort if content length grater than max size', async t => {
   t = tspl(t, { plan: 1 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     const buffer = Buffer.alloc(2 * 1024)
     res.writeHead(200, {
       'Content-Length': buffer.length,
@@ -438,7 +438,7 @@ test('Should abort if content length grater than max size', async t => {
 
 test('Should dump response body up to limit (dispatch opts)', async t => {
   t = tspl(t, { plan: 3 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     const buffer = Buffer.alloc(1 * 1024)
     res.writeHead(200, {
       'Content-Length': buffer.length,
@@ -481,7 +481,7 @@ test('Should dump response body up to limit (dispatch opts)', async t => {
 
 test('Should abort if content length grater than max size (dispatch opts)', async t => {
   t = tspl(t, { plan: 1 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     const buffer = Buffer.alloc(2 * 1024)
     res.writeHead(200, {
       'Content-Length': buffer.length,
diff --git a/test/interceptors/redirect-issue-3803.js b/test/interceptors/redirect-issue-3803.js
index a66bfa237a9..ee5d72591a4 100644
--- a/test/interceptors/redirect-issue-3803.js
+++ b/test/interceptors/redirect-issue-3803.js
@@ -9,7 +9,7 @@ const { tspl } = require('@matteo.collina/tspl')
 test('redirecting works with a FormData body', async (t) => {
   const plan = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (req.url === '/1') {
       res.writeHead(302, undefined, { location: '/2' })
       res.end()
diff --git a/test/interceptors/redirect.js b/test/interceptors/redirect.js
index c337f1f45ae..59db5d3d90c 100644
--- a/test/interceptors/redirect.js
+++ b/test/interceptors/redirect.js
@@ -752,7 +752,7 @@ test('should redirect to relative URL according to RFC 7231', async t => {
 test('Cross-origin redirects clear forbidden headers', async (t) => {
   const { strictEqual } = tspl(t, { plan: 6 })
 
-  const server1 = createServer((req, res) => {
+  const server1 = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     strictEqual(req.headers.cookie, undefined)
     strictEqual(req.headers.authorization, undefined)
     strictEqual(req.headers['proxy-authorization'], undefined)
@@ -760,7 +760,7 @@ test('Cross-origin redirects clear forbidden headers', async (t) => {
     res.end('redirected')
   }).listen(0)
 
-  const server2 = createServer((req, res) => {
+  const server2 = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     strictEqual(req.headers.authorization, 'test')
     strictEqual(req.headers.cookie, 'ddd=dddd')
 
diff --git a/test/interceptors/response-error.js b/test/interceptors/response-error.js
index 157d5130b2f..925cd96e8ba 100644
--- a/test/interceptors/response-error.js
+++ b/test/interceptors/response-error.js
@@ -8,7 +8,7 @@ const { interceptors, Client } = require('../..')
 const { responseError } = interceptors
 
 test('should throw error for error response', async () => {
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
 
   server.on('request', (req, res) => {
     res.writeHead(400, { 'content-type': 'text/plain' })
@@ -49,7 +49,7 @@ test('should throw error for error response', async () => {
 })
 
 test('should not throw error for ok response', async () => {
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
 
   server.on('request', (req, res) => {
     res.writeHead(200, { 'content-type': 'text/plain' })
@@ -84,7 +84,7 @@ test('should not throw error for ok response', async () => {
 })
 
 test('should throw error for error response, parsing JSON', async () => {
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
 
   server.on('request', (req, res) => {
     res.writeHead(400, { 'content-type': 'application/json; charset=utf-8' })
@@ -127,7 +127,7 @@ test('should throw error for error response, parsing JSON', async () => {
 })
 
 test('should throw error for error response, parsing JSON without charset', async () => {
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
 
   server.on('request', (req, res) => {
     res.writeHead(400, { 'content-type': 'application/json' })
@@ -195,7 +195,7 @@ test('should throw error for networking errors response', async () => {
 })
 
 test('should throw error for error response without content type', async () => {
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
 
   server.on('request', (req, res) => {
     res.writeHead(400, {})
diff --git a/test/interceptors/retry.js b/test/interceptors/retry.js
index 9ec033d09da..b97aa36d099 100644
--- a/test/interceptors/retry.js
+++ b/test/interceptors/retry.js
@@ -13,7 +13,7 @@ test('Should retry status code', async t => {
   t = tspl(t, { plan: 4 })
 
   let counter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const retryOptions = {
     retry: (err, { state, opts }, done) => {
       counter++
@@ -122,7 +122,7 @@ test('Should use retry-after header for retries', async t => {
   t = tspl(t, { plan: 3 })
 
   let counter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   let checkpoint
   const dispatchOptions = {
     method: 'PUT',
@@ -178,7 +178,7 @@ test('Should use retry-after header for retries (date)', async t => {
   t = tspl(t, { plan: 3 })
 
   let counter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   let checkpoint
   const requestOptions = {
     method: 'PUT',
@@ -236,7 +236,7 @@ test('Should retry with defaults', async t => {
   t = tspl(t, { plan: 2 })
 
   let counter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/',
@@ -290,7 +290,7 @@ test('Should retry with defaults', async t => {
 test('Should pass context from other interceptors', async t => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'GET',
     path: '/'
@@ -329,7 +329,7 @@ test('Should handle 206 partial content', async t => {
 
   // Took from: https://github.com/nxtedition/nxt-lib/blob/4b001ebc2f22cf735a398f35ff800dd553fe5933/test/undici/retry.js#L47
   let x = 0
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (x === 0) {
       t.ok(true, 'pass')
       res.setHeader('content-length', '6')
@@ -397,7 +397,7 @@ test('Should handle 206 partial content - bad-etag', async t => {
 
   // Took from: https://github.com/nxtedition/nxt-lib/blob/4b001ebc2f22cf735a398f35ff800dd553fe5933/test/undici/retry.js#L47
   let x = 0
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (x === 0) {
       t.ok(true, 'pass')
       res.setHeader('etag', 'asd')
@@ -463,7 +463,7 @@ test('Should handle 206 partial content - bad-etag', async t => {
 test('retrying a request with a body', async t => {
   t = tspl(t, { plan: 2 })
   let counter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestOptions = {
     method: 'POST',
     path: '/',
@@ -528,7 +528,7 @@ test('retrying a request with a body', async t => {
 test('should not error if request is not meant to be retried', async t => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.on('request', (req, res) => {
     res.writeHead(400)
     res.end('Bad request')
diff --git a/test/issue-2065.js b/test/issue-2065.js
index 9037fb557c3..2d17b879cfe 100644
--- a/test/issue-2065.js
+++ b/test/issue-2065.js
@@ -10,7 +10,7 @@ const { File } = require('node:buffer')
 test('undici.request with a FormData body should set content-length header', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.ok(req.headers['content-length'])
     res.end()
   }).listen(0)
diff --git a/test/issue-2590.js b/test/issue-2590.js
index 1da0b23f20a..e24fe4e953b 100644
--- a/test/issue-2590.js
+++ b/test/issue-2590.js
@@ -8,7 +8,7 @@ const { once } = require('node:events')
 
 test('aborting request with custom reason', async (t) => {
   t = tspl(t, { plan: 3 })
-  const server = createServer(() => {}).listen(0)
+  const server = createServer({ joinDuplicateHeaders: true }, () => {}).listen(0)
 
   after(() => server.close())
   await once(server, 'listening')
diff --git a/test/issue-3356.js b/test/issue-3356.js
index 1f671fbe3e1..40cecde1386 100644
--- a/test/issue-3356.js
+++ b/test/issue-3356.js
@@ -12,7 +12,7 @@ test('https://github.com/nodejs/undici/issues/3356', { skip: process.env.CITGM }
   t = tspl(t, { plan: 3 })
 
   let shouldRetry = true
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.on('request', (req, res) => {
     res.writeHead(200, { 'content-type': 'text/plain' })
     if (shouldRetry) {
diff --git a/test/issue-3934.js b/test/issue-3934.js
index 19c1544c213..3898f1144c6 100644
--- a/test/issue-3934.js
+++ b/test/issue-3934.js
@@ -8,7 +8,7 @@ const { Agent, RetryAgent, request } = require('..')
 
 // https://github.com/nodejs/undici/issues/3934
 test('WrapHandler works with multiple header values', async (t) => {
-  const server = createServer(async (_req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (_req, res) => {
     const headers = [
       ['set-cookie', 'a'],
       ['set-cookie', 'b'],
diff --git a/test/issue-3959.js b/test/issue-3959.js
index 2d83a6030e0..59bfab50789 100644
--- a/test/issue-3959.js
+++ b/test/issue-3959.js
@@ -8,7 +8,7 @@ const { interceptors } = require('..')
 describe('Cache with Vary headers', () => {
   async function runCacheTest (store) {
     let requestCount = 0
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       requestCount++
       res.setHeader('Vary', 'Accept-Encoding')
       res.setHeader('Cache-Control', 'max-age=60')
diff --git a/test/issue-803.js b/test/issue-803.js
index 5c2eb21b6e5..12540100f8d 100644
--- a/test/issue-803.js
+++ b/test/issue-803.js
@@ -10,7 +10,7 @@ test('https://github.com/nodejs/undici/issues/803', { timeout: 60000 }, async (t
   t = tspl(t, { plan: 2 })
   const SIZE = 5900373096
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     const chunkSize = res.writableHighWaterMark << 5
     const parts = (SIZE / chunkSize) | 0
     const lastPartSize = SIZE % chunkSize
diff --git a/test/jest/instanceof-error.test.js b/test/jest/instanceof-error.test.js
index 30667262dee..c93b81d6e42 100644
--- a/test/jest/instanceof-error.test.js
+++ b/test/jest/instanceof-error.test.js
@@ -23,7 +23,7 @@ it('Real use-case', async () => {
   const ac = new AbortController()
   ac.abort()
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   }).listen(0)
 
diff --git a/test/jest/test.js b/test/jest/test.js
index 4be7556e27e..02fed2d902e 100644
--- a/test/jest/test.js
+++ b/test/jest/test.js
@@ -5,7 +5,7 @@ const { createServer } = require('node:http')
 /* global test, expect */
 
 test('should work in jest', async () => {
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     expect(req.url).toBe('/')
     expect(req.method).toBe('POST')
     expect(req.headers.host).toBe(`localhost:${server.address().port}`)
diff --git a/test/max-headers.js b/test/max-headers.js
index 0f1422a60b4..9edd8d1efdc 100644
--- a/test/max-headers.js
+++ b/test/max-headers.js
@@ -14,7 +14,7 @@ test('handle a lot of headers', async (t) => {
     headers[n] = String(n)
   }
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200, headers)
     res.end()
   })
diff --git a/test/max-response-size.js b/test/max-response-size.js
index 1e0d904469a..648a4cb08a0 100644
--- a/test/max-response-size.js
+++ b/test/max-response-size.js
@@ -9,7 +9,7 @@ describe('max response size', async (t) => {
   test('default max default size should allow all responses', async (t) => {
     t = tspl(t, { plan: 3 })
 
-    const server = createServer()
+    const server = createServer({ joinDuplicateHeaders: true })
     after(() => server.close())
 
     server.on('request', (req, res) => {
@@ -39,7 +39,7 @@ describe('max response size', async (t) => {
   test('max response size set to zero should allow only empty responses', async (t) => {
     t = tspl(t, { plan: 3 })
 
-    const server = createServer()
+    const server = createServer({ joinDuplicateHeaders: true })
     after(() => server.close())
 
     server.on('request', (req, res) => {
@@ -69,7 +69,7 @@ describe('max response size', async (t) => {
   test('should throw an error if the response is too big', async (t) => {
     t = tspl(t, { plan: 3 })
 
-    const server = createServer()
+    const server = createServer({ joinDuplicateHeaders: true })
     after(() => server.close())
 
     server.on('request', (req, res) => {
diff --git a/test/mock-agent.js b/test/mock-agent.js
index aac6585e5e8..5f6488f04eb 100644
--- a/test/mock-agent.js
+++ b/test/mock-agent.js
@@ -269,7 +269,7 @@ test('MockAgent - .close should clean up registered clients', async (t) => {
 test('MockAgent - [kClients] should match encapsulated agent', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -299,7 +299,7 @@ test('MockAgent - [kClients] should match encapsulated agent', async (t) => {
 test('MockAgent - basic intercept with MockAgent.request', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -343,7 +343,7 @@ test('MockAgent - basic intercept with MockAgent.request', async (t) => {
 test('MockAgent - basic intercept with request', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -386,7 +386,7 @@ test('MockAgent - basic intercept with request', async (t) => {
 test('MockAgent - should support local agents', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -432,7 +432,7 @@ test('MockAgent - should support local agents', async (t) => {
 test('MockAgent - should support specifying custom agents to mock', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -479,7 +479,7 @@ test('MockAgent - should support specifying custom agents to mock', async (t) =>
 test('MockAgent - basic Client intercept with request', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -524,7 +524,7 @@ test('MockAgent - basic Client intercept with request', async (t) => {
 test('MockAgent - basic intercept with multiple pools', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -576,7 +576,7 @@ test('MockAgent - basic intercept with multiple pools', async (t) => {
 test('MockAgent - should handle multiple responses for an interceptor', async (t) => {
   t = tspl(t, { plan: 6 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -639,7 +639,7 @@ test('MockAgent - should handle multiple responses for an interceptor', async (t
 test('MockAgent - should call original Pool dispatch if request not found', async (t) => {
   t = tspl(t, { plan: 5 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.url, '/foo')
     t.strictEqual(req.method, 'GET')
     res.setHeader('content-type', 'text/plain')
@@ -668,7 +668,7 @@ test('MockAgent - should call original Pool dispatch if request not found', asyn
 test('MockAgent - should call original Client dispatch if request not found', async (t) => {
   t = tspl(t, { plan: 5 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.url, '/foo')
     t.strictEqual(req.method, 'GET')
     res.setHeader('content-type', 'text/plain')
@@ -697,7 +697,7 @@ test('MockAgent - should call original Client dispatch if request not found', as
 test('MockAgent - should handle string responses', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -761,7 +761,7 @@ test('MockAgent - should handle basic concurrency for requests', { jobs: 5 }, as
 test('MockAgent - handle delays to simulate work', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -799,7 +799,7 @@ test('MockAgent - handle delays to simulate work', async (t) => {
 test('MockAgent - should persist requests', async (t) => {
   t = tspl(t, { plan: 8 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -1060,7 +1060,7 @@ test('MockAgent - clearCallHistory should clear call history logs', async (t) =>
 test('MockAgent - handle persists with delayed requests', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -1106,7 +1106,7 @@ test('MockAgent - handle persists with delayed requests', async (t) => {
 test('MockAgent - calling close on a mock pool should not affect other mock pools', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -1164,7 +1164,7 @@ test('MockAgent - calling close on a mock pool should not affect other mock pool
 test('MockAgent - close removes all registered mock clients', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -1220,7 +1220,7 @@ test('MockAgent - close clear all registered mock call history logs', async (t)
 test('MockAgent - close removes all registered mock pools', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -1254,7 +1254,7 @@ test('MockAgent - close removes all registered mock pools', async (t) => {
 test('MockAgent - should handle replyWithError', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -1282,7 +1282,7 @@ test('MockAgent - should handle replyWithError', async (t) => {
 test('MockAgent - should support setting a reply to respond a set amount of times', async (t) => {
   t = tspl(t, { plan: 9 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.url, '/foo')
     t.strictEqual(req.method, 'GET')
     res.setHeader('content-type', 'text/plain')
@@ -1333,7 +1333,7 @@ test('MockAgent - should support setting a reply to respond a set amount of time
 test('MockAgent - persist overrides times', async (t) => {
   t = tspl(t, { plan: 6 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -1389,7 +1389,7 @@ test('MockAgent - persist overrides times', async (t) => {
 test('MockAgent - matcher should not find mock dispatch if path is of unsupported type', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.url, '/foo')
     t.strictEqual(req.method, 'GET')
     res.end('hello')
@@ -1422,7 +1422,7 @@ test('MockAgent - matcher should not find mock dispatch if path is of unsupporte
 test('MockAgent - should match path with regex', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -1468,7 +1468,7 @@ test('MockAgent - should match path with regex', async (t) => {
 test('MockAgent - should match path with function', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -1502,7 +1502,7 @@ test('MockAgent - should match path with function', async (t) => {
 test('MockAgent - should match method with regex', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -1536,7 +1536,7 @@ test('MockAgent - should match method with regex', async (t) => {
 test('MockAgent - should match method with function', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -1570,7 +1570,7 @@ test('MockAgent - should match method with function', async (t) => {
 test('MockAgent - should match body with regex', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -1606,7 +1606,7 @@ test('MockAgent - should match body with regex', async (t) => {
 test('MockAgent - should match body with function', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -1642,7 +1642,7 @@ test('MockAgent - should match body with function', async (t) => {
 test('MockAgent - should match headers with string', async (t) => {
   t = tspl(t, { plan: 6 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('should not be called')
     t.fail('should not be called')
     t.end()
@@ -1715,7 +1715,7 @@ test('MockAgent - should match headers with string', async (t) => {
 test('MockAgent - should match headers with regex', async (t) => {
   t = tspl(t, { plan: 6 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('should not be called')
     t.fail('should not be called')
     t.end()
@@ -1788,7 +1788,7 @@ test('MockAgent - should match headers with regex', async (t) => {
 test('MockAgent - should match headers with function', async (t) => {
   t = tspl(t, { plan: 6 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('should not be called')
     t.fail('should not be called')
     t.end()
@@ -1861,7 +1861,7 @@ test('MockAgent - should match headers with function', async (t) => {
 test('MockAgent - should match url with regex', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -1895,7 +1895,7 @@ test('MockAgent - should match url with regex', async (t) => {
 test('MockAgent - should match url with function', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -1929,7 +1929,7 @@ test('MockAgent - should match url with function', async (t) => {
 test('MockAgent - handle default reply headers', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -1967,7 +1967,7 @@ test('MockAgent - handle default reply headers', async (t) => {
 test('MockAgent - handle default reply trailers', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -2005,7 +2005,7 @@ test('MockAgent - handle default reply trailers', async (t) => {
 test('MockAgent - return calculated content-length if specified', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -2043,7 +2043,7 @@ test('MockAgent - return calculated content-length if specified', async (t) => {
 test('MockAgent - return calculated content-length for object response if specified', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -2081,7 +2081,7 @@ test('MockAgent - return calculated content-length for object response if specif
 test('MockAgent - should activate and deactivate mock clients', async (t) => {
   t = tspl(t, { plan: 9 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.url, '/foo')
     t.strictEqual(req.method, 'GET')
     res.setHeader('content-type', 'text/plain')
@@ -2142,7 +2142,7 @@ test('MockAgent - should activate and deactivate mock clients', async (t) => {
 test('MockAgent - enableNetConnect should allow all original dispatches to be called if dispatch not found', async (t) => {
   t = tspl(t, { plan: 5 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.url, '/foo')
     t.strictEqual(req.method, 'GET')
     res.setHeader('content-type', 'text/plain')
@@ -2179,7 +2179,7 @@ test('MockAgent - enableNetConnect should allow all original dispatches to be ca
 test('MockAgent - enableNetConnect with a host string should allow all original dispatches to be called if mockDispatch not found', async (t) => {
   t = tspl(t, { plan: 5 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.url, '/foo')
     t.strictEqual(req.method, 'GET')
     res.setHeader('content-type', 'text/plain')
@@ -2216,7 +2216,7 @@ test('MockAgent - enableNetConnect with a host string should allow all original
 test('MockAgent - enableNetConnect when called with host string multiple times should allow all original dispatches to be called if mockDispatch not found', async (t) => {
   t = tspl(t, { plan: 5 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.url, '/foo')
     t.strictEqual(req.method, 'GET')
     res.setHeader('content-type', 'text/plain')
@@ -2254,7 +2254,7 @@ test('MockAgent - enableNetConnect when called with host string multiple times s
 test('MockAgent - enableNetConnect with a host regex should allow all original dispatches to be called if mockDispatch not found', async (t) => {
   t = tspl(t, { plan: 5 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.url, '/foo')
     t.strictEqual(req.method, 'GET')
     res.setHeader('content-type', 'text/plain')
@@ -2291,7 +2291,7 @@ test('MockAgent - enableNetConnect with a host regex should allow all original d
 test('MockAgent - enableNetConnect with a function should allow all original dispatches to be called if mockDispatch not found', async (t) => {
   t = tspl(t, { plan: 5 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.url, '/foo')
     t.strictEqual(req.method, 'GET')
     res.setHeader('content-type', 'text/plain')
@@ -2344,7 +2344,7 @@ test('MockAgent - enableNetConnect with an unknown input should throw', async (t
 test('MockAgent - enableNetConnect should throw if dispatch not matched for path and the origin was not allowed by net connect', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.fail('should not be called')
     t.end()
     res.end('should not be called')
@@ -2375,7 +2375,7 @@ test('MockAgent - enableNetConnect should throw if dispatch not matched for path
 test('MockAgent - enableNetConnect should throw if dispatch not matched for method and the origin was not allowed by net connect', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.fail('should not be called')
     t.end()
     res.end('should not be called')
@@ -2406,7 +2406,7 @@ test('MockAgent - enableNetConnect should throw if dispatch not matched for meth
 test('MockAgent - enableNetConnect should throw if dispatch not matched for body and the origin was not allowed by net connect', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.fail('should not be called')
     t.end()
     res.end('should not be called')
@@ -2439,7 +2439,7 @@ test('MockAgent - enableNetConnect should throw if dispatch not matched for body
 test('MockAgent - enableNetConnect should throw if dispatch not matched for headers and the origin was not allowed by net connect', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.fail('should not be called')
     t.end()
     res.end('should not be called')
@@ -2476,7 +2476,7 @@ test('MockAgent - enableNetConnect should throw if dispatch not matched for head
 test('MockAgent - disableNetConnect should throw if dispatch not found by net connect', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(req.url, '/foo')
     t.strictEqual(req.method, 'GET')
     res.setHeader('content-type', 'text/plain')
@@ -2508,7 +2508,7 @@ test('MockAgent - disableNetConnect should throw if dispatch not found by net co
 test('MockAgent - headers function interceptor', async (t) => {
   t = tspl(t, { plan: 8 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.fail('should not be called')
     t.end()
     res.end('should not be called')
@@ -2570,7 +2570,7 @@ test('MockAgent - clients are not garbage collected', async (t) => {
   const samples = 250
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.fail('should not be called')
     t.end()
     res.end('should not be called')
@@ -2833,7 +2833,7 @@ test('MockAgent - Sending ReadableStream body', async (t) => {
   const mockAgent = new MockAgent()
   setGlobalDispatcher(mockAgent)
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     req.pipe(res)
   })
diff --git a/test/mock-client.js b/test/mock-client.js
index 9b8f416ec34..fd77f0047b8 100644
--- a/test/mock-client.js
+++ b/test/mock-client.js
@@ -198,7 +198,7 @@ test('MockClient - close should run without error', async (t) => {
 test('MockClient - should be able to set as globalDispatcher', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -234,7 +234,7 @@ test('MockClient - should be able to set as globalDispatcher', async (t) => {
 test('MockClient - should support query params', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -275,7 +275,7 @@ test('MockClient - should support query params', async (t) => {
 test('MockClient - should intercept query params with hardcoded path', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -315,7 +315,7 @@ test('MockClient - should intercept query params with hardcoded path', async (t)
 test('MockClient - should intercept query params regardless of key ordering', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -363,7 +363,7 @@ test('MockClient - should intercept query params regardless of key ordering', as
 test('MockClient - should be able to use as a local dispatcher', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -399,7 +399,7 @@ test('MockClient - should be able to use as a local dispatcher', async (t) => {
 test('MockClient - basic intercept with MockClient.request', async (t) => {
   t = tspl(t, { plan: 5 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
diff --git a/test/mock-pool.js b/test/mock-pool.js
index b52d3406cce..4e079451741 100644
--- a/test/mock-pool.js
+++ b/test/mock-pool.js
@@ -184,7 +184,7 @@ test('MockPool - close should run without error', async (t) => {
 test('MockPool - should be able to set as globalDispatcher', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -220,7 +220,7 @@ test('MockPool - should be able to set as globalDispatcher', async (t) => {
 test('MockPool - should be able to use as a local dispatcher', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
@@ -256,7 +256,7 @@ test('MockPool - should be able to use as a local dispatcher', async (t) => {
 test('MockPool - basic intercept with MockPool.request', async (t) => {
   t = tspl(t, { plan: 5 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('should not be called')
     t.fail('should not be called')
diff --git a/test/no-strict-content-length.js b/test/no-strict-content-length.js
index aa3d1725cb2..4779bfcd862 100644
--- a/test/no-strict-content-length.js
+++ b/test/no-strict-content-length.js
@@ -28,7 +28,7 @@ describe('strictContentLength: false', () => {
   test('request invalid content-length', async (t) => {
     t = tspl(t, { plan: 8 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end()
     })
     after(() => server.close())
@@ -146,7 +146,7 @@ describe('strictContentLength: false', () => {
   test('request streaming content-length less than body size', async (t) => {
     t = tspl(t, { plan: 1 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end()
     })
     after(() => server.close())
@@ -183,7 +183,7 @@ describe('strictContentLength: false', () => {
   test('request streaming content-length greater than body size', async (t) => {
     t = tspl(t, { plan: 1 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end()
     })
     after(() => server.close())
@@ -220,7 +220,7 @@ describe('strictContentLength: false', () => {
   test('request streaming data when content-length=0', async (t) => {
     t = tspl(t, { plan: 1 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end()
     })
     after(() => server.close())
@@ -257,7 +257,7 @@ describe('strictContentLength: false', () => {
   test('request async iterating content-length less than body size', async (t) => {
     t = tspl(t, { plan: 1 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end()
     })
     after(() => server.close())
@@ -294,7 +294,7 @@ describe('strictContentLength: false', () => {
   test('request async iterator content-length greater than body size', async (t) => {
     t = tspl(t, { plan: 1 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end()
     })
     after(() => server.close())
@@ -330,7 +330,7 @@ describe('strictContentLength: false', () => {
   test('request async iterator data when content-length=0', async (t) => {
     t = tspl(t, { plan: 1 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end()
     })
     after(() => server.close())
diff --git a/test/node-fetch/main.js b/test/node-fetch/main.js
index 1473caf198c..149292b2a56 100644
--- a/test/node-fetch/main.js
+++ b/test/node-fetch/main.js
@@ -1621,7 +1621,7 @@ describe('node-fetch', () => {
 
   it('should support http request', { timeout: 5000 }, async function (t) {
     t = tspl(t, { plan: 2 })
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end()
     })
     after(() => server.close())
diff --git a/test/node-fetch/utils/server.js b/test/node-fetch/utils/server.js
index 233c30da106..a6cfb2d3459 100644
--- a/test/node-fetch/utils/server.js
+++ b/test/node-fetch/utils/server.js
@@ -7,7 +7,7 @@ const Busboy = require('@fastify/busboy')
 
 module.exports = class TestServer {
   constructor () {
-    this.server = http.createServer(this.router)
+    this.server = http.createServer({ joinDuplicateHeaders: true }, this.router)
     // Node 8 default keepalive timeout is 5000ms
     // make it shorter here as we want to close server quickly at the end of tests
     this.server.keepAliveTimeout = 1000
diff --git a/test/node-test/abort-controller.js b/test/node-test/abort-controller.js
index 6db151e5d16..f8249c13039 100644
--- a/test/node-test/abort-controller.js
+++ b/test/node-test/abort-controller.js
@@ -24,7 +24,7 @@ for (const { AbortControllerImpl, controllerName } of controllers) {
   test(`Abort ${controllerName} before creating request`, async (t) => {
     const p = tspl(t, { plan: 1 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       p.fail()
     })
     t.after(closeServerAsPromise(server))
@@ -48,7 +48,7 @@ for (const { AbortControllerImpl, controllerName } of controllers) {
     const p = tspl(t, { plan: 3 })
 
     let count = 0
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       if (count === 1) {
         p.fail('The second request should never be executed')
       }
@@ -87,7 +87,7 @@ for (const { AbortControllerImpl, controllerName } of controllers) {
     const p = tspl(t, { plan: 1 })
 
     const abortController = new AbortControllerImpl()
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       abortController.abort()
       res.setHeader('content-type', 'text/plain')
       res.end('hello world')
@@ -110,7 +110,7 @@ for (const { AbortControllerImpl, controllerName } of controllers) {
     const p = tspl(t, { plan: 1 })
 
     const abortController = new AbortControllerImpl()
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(200, { 'content-type': 'text/plain' })
       res.flushHeaders()
       abortController.abort()
@@ -134,7 +134,7 @@ for (const { AbortControllerImpl, controllerName } of controllers) {
     const p = tspl(t, { plan: 2 })
 
     const abortController = new AbortControllerImpl()
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(200, { 'content-type': 'text/plain' })
       res.write('hello')
     })
@@ -163,7 +163,7 @@ for (const { AbortControllerImpl, controllerName } of controllers) {
       const p = tspl(t, { plan: 1 })
 
       const abortController = new AbortControllerImpl()
-      const server = createServer((req, res) => {
+      const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
         abortController.abort()
         res.setHeader('content-type', 'text/plain')
         res.end('hello world')
@@ -192,7 +192,7 @@ for (const { AbortControllerImpl, controllerName } of controllers) {
       const p = tspl(t, { plan: 1 })
 
       const abortController = new AbortControllerImpl()
-      const server = createServer((req, res) => {
+      const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
         res.writeHead(200, { 'content-type': 'text/plain' })
         res.flushHeaders()
         abortController.abort()
@@ -222,7 +222,7 @@ for (const { AbortControllerImpl, controllerName } of controllers) {
       const p = tspl(t, { plan: 2 })
 
       const abortController = new AbortControllerImpl()
-      const server = createServer((req, res) => {
+      const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
         res.writeHead(200, { 'content-type': 'text/plain' })
         res.write('hello')
       })
diff --git a/test/node-test/abort-event-emitter.js b/test/node-test/abort-event-emitter.js
index ad7231331cb..6f7a010f649 100644
--- a/test/node-test/abort-event-emitter.js
+++ b/test/node-test/abort-event-emitter.js
@@ -14,7 +14,7 @@ test('Abort before sending request (no body)', async (t) => {
   const p = tspl(t, { plan: 4 })
 
   let count = 0
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (count === 1) {
       p.fail('The second request should never be executed')
     }
@@ -63,7 +63,7 @@ test('Abort before sending request (no body) async iterator', async (t) => {
   const p = tspl(t, { plan: 3 })
 
   let count = 0
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (count === 1) {
       t.fail('The second request should never be executed')
     }
@@ -109,7 +109,7 @@ test('Abort while waiting response (no body)', async (t) => {
   const p = tspl(t, { plan: 1 })
 
   const ee = new EventEmitter()
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     ee.emit('abort')
     res.setHeader('content-type', 'text/plain')
     res.end('hello world')
@@ -132,7 +132,7 @@ test('Abort while waiting response (write headers started) (no body)', async (t)
   const p = tspl(t, { plan: 1 })
 
   const ee = new EventEmitter()
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200, { 'content-type': 'text/plain' })
     res.flushHeaders()
     ee.emit('abort')
@@ -156,7 +156,7 @@ test('Abort while waiting response (write headers and write body started) (no bo
   const p = tspl(t, { plan: 2 })
 
   const ee = new EventEmitter()
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200, { 'content-type': 'text/plain' })
     res.write('hello')
   })
@@ -184,7 +184,7 @@ function waitingWithBody (body, type) {
     const p = tspl(t, { plan: 1 })
 
     const ee = new EventEmitter()
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       ee.emit('abort')
       res.setHeader('content-type', 'text/plain')
       res.end('hello world')
@@ -213,7 +213,7 @@ function writeHeadersStartedWithBody (body, type) {
     const p = tspl(t, { plan: 1 })
 
     const ee = new EventEmitter()
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(200, { 'content-type': 'text/plain' })
       res.flushHeaders()
       ee.emit('abort')
@@ -243,7 +243,7 @@ function writeBodyStartedWithBody (body, type) {
     const p = tspl(t, { plan: 2 })
 
     const ee = new EventEmitter()
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.writeHead(200, { 'content-type': 'text/plain' })
       res.write('hello')
     })
diff --git a/test/node-test/agent.js b/test/node-test/agent.js
index c5238d7b570..6f7d7ffe53c 100644
--- a/test/node-test/agent.js
+++ b/test/node-test/agent.js
@@ -45,7 +45,7 @@ test('agent should call callback after closing internal pools', async (t) => {
 
   const wanted = 'payload'
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('Content-Type', 'text/plain')
     res.end(wanted)
   })
@@ -97,7 +97,7 @@ test('agent should close internal pools', async (t) => {
 
   const wanted = 'payload'
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('Content-Type', 'text/plain')
     res.end(wanted)
   })
@@ -138,7 +138,7 @@ test('agent should destroy internal pools and call callback', async (t) => {
 
   const wanted = 'payload'
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('Content-Type', 'text/plain')
     res.end(wanted)
   })
@@ -200,7 +200,7 @@ test('agent should destroy internal pools', async t => {
 
   const wanted = 'payload'
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('Content-Type', 'text/plain')
     res.end(wanted)
   })
@@ -239,7 +239,7 @@ test('multiple connections', async t => {
   const connections = 3
   const p = tspl(t, { plan: 6 * connections })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200, {
       Connection: 'keep-alive',
       'Keep-Alive': 'timeout=1s'
@@ -299,7 +299,7 @@ test('agent factory supports URL parameter', async (t) => {
     }
   })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('Content-Type', 'text/plain')
     res.end('asd')
   })
@@ -337,7 +337,7 @@ test('agent factory supports string parameter', async (t) => {
     }
   })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('Content-Type', 'text/plain')
     res.end('asd')
   })
@@ -357,7 +357,7 @@ test('with globalAgent', async t => {
   const p = tspl(t, { plan: 6 })
   const wanted = 'payload'
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     p.strictEqual('/', req.url)
     p.strictEqual('GET', req.method)
     p.strictEqual(`localhost:${server.address().port}`, req.headers.host)
@@ -392,7 +392,7 @@ test('with local agent', async t => {
   const p = tspl(t, { plan: 6 })
   const wanted = 'payload'
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     p.strictEqual('/', req.url)
     p.strictEqual('GET', req.method)
     p.strictEqual(`localhost:${server.address().port}`, req.headers.host)
@@ -443,7 +443,7 @@ test('with globalAgent', async t => {
   const p = tspl(t, { plan: 6 })
   const wanted = 'payload'
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     p.strictEqual('/', req.url)
     p.strictEqual('GET', req.method)
     p.strictEqual(`localhost:${server.address().port}`, req.headers.host)
@@ -484,7 +484,7 @@ test('with a local agent', async t => {
   const p = tspl(t, { plan: 6 })
   const wanted = 'payload'
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     p.strictEqual('/', req.url)
     p.strictEqual('GET', req.method)
     p.strictEqual(`localhost:${server.address().port}`, req.headers.host)
@@ -544,7 +544,7 @@ test('with globalAgent', async t => {
   const p = tspl(t, { plan: 6 })
   const wanted = 'payload'
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     p.strictEqual('/', req.url)
     p.strictEqual('GET', req.method)
     p.strictEqual(`localhost:${server.address().port}`, req.headers.host)
@@ -585,7 +585,7 @@ test('with a local agent', async t => {
   const p = tspl(t, { plan: 6 })
   const wanted = 'payload'
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     p.strictEqual('/', req.url)
     p.strictEqual('GET', req.method)
     p.strictEqual(`localhost:${server.address().port}`, req.headers.host)
@@ -681,7 +681,7 @@ test('dispatch validations', async t => {
     }
   }
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('Content-Type', 'text/plain')
     res.end('asd')
   })
@@ -726,7 +726,7 @@ test('drain', async t => {
     }
   }
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('Content-Type', 'text/plain')
     res.end('asd')
   })
@@ -747,7 +747,7 @@ test('drain', async t => {
 test('global api', async t => {
   const p = tspl(t, { plan: 6 * 2 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (req.url === '/bar') {
       p.strictEqual(req.method, 'PUT')
       p.strictEqual(req.url, '/bar')
diff --git a/test/node-test/async_hooks.js b/test/node-test/async_hooks.js
index 63d42dd1082..a454266a880 100644
--- a/test/node-test/async_hooks.js
+++ b/test/node-test/async_hooks.js
@@ -37,7 +37,7 @@ hook.enable()
 test('async hooks', async (t) => {
   const p = tspl(t, { plan: 31 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     readFile(__filename, (err, buf) => {
       p.ifError(err)
@@ -148,7 +148,7 @@ test('async hooks', async (t) => {
 test('async hooks client is destroyed', async (t) => {
   const p = tspl(t, { plan: 7 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     readFile(__filename, (err, buf) => {
       p.ifError(err)
@@ -187,7 +187,7 @@ test('async hooks client is destroyed', async (t) => {
 test('async hooks pipeline handler', async (t) => {
   const p = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('hello')
   })
   t.after(closeServerAsPromise(server))
diff --git a/test/node-test/autoselectfamily.js b/test/node-test/autoselectfamily.js
index 4baf1ab0aa1..1314a328676 100644
--- a/test/node-test/autoselectfamily.js
+++ b/test/node-test/autoselectfamily.js
@@ -67,7 +67,7 @@ test('with autoSelectFamily enable the request succeeds when using request', { s
   const p = tspl(t, { plan: 3 })
 
   createDnsServer('::1', '127.0.0.1', function (_, { dnsServer, lookup }) {
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end('hello')
     })
 
@@ -107,7 +107,7 @@ test('with autoSelectFamily enable the request succeeds when using a client', {
   const p = tspl(t, { plan: 3 })
 
   createDnsServer('::1', '127.0.0.1', function (_, { dnsServer, lookup }) {
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end('hello')
     })
 
@@ -148,7 +148,7 @@ test('with autoSelectFamily disabled the request fails when using request', { sk
   const p = tspl(t, { plan: 1 })
 
   createDnsServer('::1', '127.0.0.1', function (_, { dnsServer, lookup }) {
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end('hello')
     })
 
@@ -176,7 +176,7 @@ test('with autoSelectFamily disabled via Agent.Options["autoSelectFamily"] the r
   const p = tspl(t, { plan: 1 })
 
   createDnsServer('::1', '127.0.0.1', function (_, { dnsServer, lookup }) {
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end('hello')
     })
 
@@ -204,7 +204,7 @@ test('with autoSelectFamily disabled the request fails when using a client', { s
   const p = tspl(t, { plan: 1 })
 
   createDnsServer('::1', '127.0.0.1', function (_, { dnsServer, lookup }) {
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end('hello')
     })
 
@@ -233,7 +233,7 @@ test('with autoSelectFamily disabled via Client.Options["autoSelectFamily"] the
   const p = tspl(t, { plan: 1 })
 
   createDnsServer('::1', '127.0.0.1', function (_, { dnsServer, lookup }) {
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end('hello')
     })
 
diff --git a/test/node-test/balanced-pool.js b/test/node-test/balanced-pool.js
index 725c31ed190..d2f3bdd0bb1 100644
--- a/test/node-test/balanced-pool.js
+++ b/test/node-test/balanced-pool.js
@@ -52,7 +52,7 @@ test('basic get', async (t) => {
   const p = tspl(t, { plan: 16 })
 
   let server1Called = 0
-  const server1 = createServer((req, res) => {
+  const server1 = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     server1Called++
     p.strictEqual('/', req.url)
     p.strictEqual('GET', req.method)
@@ -64,7 +64,7 @@ test('basic get', async (t) => {
   await promisify(server1.listen).call(server1, 0)
 
   let server2Called = 0
-  const server2 = createServer((req, res) => {
+  const server2 = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     server2Called++
     p.strictEqual('/', req.url)
     p.strictEqual('GET', req.method)
@@ -109,7 +109,7 @@ test('connect/disconnect event(s)', async (t) => {
 
   const p = tspl(t, { plan: clients * 5 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200, {
       Connection: 'keep-alive',
       'Keep-Alive': 'timeout=1s'
@@ -151,7 +151,7 @@ test('connect/disconnect event(s)', async (t) => {
 test('busy', async (t) => {
   const p = tspl(t, { plan: 8 * 6 + 2 + 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     p.strictEqual('/', req.url)
     p.strictEqual('GET', req.method)
     res.setHeader('content-type', 'text/plain')
@@ -205,7 +205,7 @@ test('factory option with basic get request', async (t) => {
   const client = new BalancedPool([], opts)
 
   let serverCalled = 0
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     serverCalled++
     p.strictEqual('/', req.url)
     p.strictEqual('GET', req.method)
@@ -298,7 +298,7 @@ class TestServer {
   }
 
   start () {
-    this.server = createServer((req, res) => {
+    this.server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       if (this._shouldHangupOnClient()) {
         req.destroy(new Error('(ツ)'))
         return
diff --git a/test/node-test/ca-fingerprint.js b/test/node-test/ca-fingerprint.js
index bad8aeff555..54b9d7adc31 100644
--- a/test/node-test/ca-fingerprint.js
+++ b/test/node-test/ca-fingerprint.js
@@ -17,7 +17,7 @@ const caFingerprint = getFingerprint(pem.cert.toString()
 test('Validate CA fingerprint with a custom connector', async t => {
   const p = tspl(t, { plan: 2 })
 
-  const server = https.createServer(pem, (req, res) => {
+  const server = https.createServer({ ...pem, joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('Content-Type', 'text/plain')
     res.end('hello')
   })
@@ -64,7 +64,7 @@ test('Validate CA fingerprint with a custom connector', async t => {
 test('Bad CA fingerprint with a custom connector', async t => {
   const p = tspl(t, { plan: 2 })
 
-  const server = https.createServer(pem, (req, res) => {
+  const server = https.createServer({ ...pem, joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('Content-Type', 'text/plain')
     res.end('hello')
   })
diff --git a/test/node-test/client-abort.js b/test/node-test/client-abort.js
index 52395f0dbfb..cf0c6fc1553 100644
--- a/test/node-test/client-abort.js
+++ b/test/node-test/client-abort.js
@@ -11,7 +11,7 @@ class OnAbortError extends Error {}
 test('aborted response errors', async (t) => {
   const p = tspl(t, { plan: 3 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.once('request', (req, res) => {
     // TODO: res.write will cause body to emit 'error' twice
     // due to bug in readable-stream.
@@ -42,7 +42,7 @@ test('aborted response errors', async (t) => {
 test('aborted req', async (t) => {
   const p = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(Buffer.alloc(4 + 1, 'a'))
   })
   t.after(server.close.bind(server))
@@ -72,7 +72,7 @@ test('aborted req', async (t) => {
 test('abort', async (t) => {
   const p = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   t.after(server.close.bind(server))
@@ -113,7 +113,7 @@ test('abort', async (t) => {
 test('abort pipelined', async (t) => {
   const p = tspl(t, { plan: 6 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
   })
   t.after(server.close.bind(server))
 
@@ -183,7 +183,7 @@ test('abort pipelined', async (t) => {
 test('propagate unallowed throws in request.onError', async (t) => {
   const p = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   t.after(server.close.bind(server))
diff --git a/test/node-test/client-connect.js b/test/node-test/client-connect.js
index 28271d348cf..e201cedaaad 100644
--- a/test/node-test/client-connect.js
+++ b/test/node-test/client-connect.js
@@ -11,7 +11,7 @@ const { closeServerAsPromise } = require('../utils/node-http')
 test('basic connect', async (t) => {
   const p = tspl(t, { plan: 3 })
 
-  const server = http.createServer((c) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (c) => {
     p.ok(0)
   })
   server.on('connect', (req, socket, firstBodyChunk) => {
@@ -60,7 +60,7 @@ test('basic connect', async (t) => {
 test('connect error', async (t) => {
   const p = tspl(t, { plan: 1 })
 
-  const server = http.createServer((c) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (c) => {
     p.ok(0)
   })
   server.on('connect', (req, socket, firstBodyChunk) => {
@@ -115,7 +115,7 @@ test('connect wait for empty pipeline', async (t) => {
   const p = tspl(t, { plan: 7 })
 
   let canConnect = false
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
     canConnect = true
   })
@@ -184,7 +184,7 @@ test('connect wait for empty pipeline', async (t) => {
 test('connect aborted', async (t) => {
   const p = tspl(t, { plan: 6 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     p.ok(0)
   })
   server.on('connect', (req, c, firstBodyChunk) => {
@@ -225,7 +225,7 @@ test('connect aborted', async (t) => {
 test('basic connect error', async (t) => {
   const p = tspl(t, { plan: 2 })
 
-  const server = http.createServer((c) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (c) => {
     p.ok(0)
   })
   server.on('connect', (req, socket, firstBodyChunk) => {
@@ -264,7 +264,7 @@ test('basic connect error', async (t) => {
 test('connect invalid signal', async (t) => {
   const p = tspl(t, { plan: 2 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     p.ok(0)
   })
   server.on('connect', (req, c, firstBodyChunk) => {
diff --git a/test/node-test/client-dispatch.js b/test/node-test/client-dispatch.js
index 8b7991a3156..0f467f75cb4 100644
--- a/test/node-test/client-dispatch.js
+++ b/test/node-test/client-dispatch.js
@@ -94,7 +94,7 @@ test('dispatch invalid opts', (t) => {
 test('basic dispatch get', async (t) => {
   const p = tspl(t, { plan: 11 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     p.strictEqual('/', req.url)
     p.strictEqual('GET', req.method)
     p.strictEqual(`localhost:${server.address().port}`, req.headers.host)
@@ -147,7 +147,7 @@ test('basic dispatch get', async (t) => {
 test('trailers dispatch get', async (t) => {
   const p = tspl(t, { plan: 12 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     p.strictEqual('/', req.url)
     p.strictEqual('GET', req.method)
     p.strictEqual(`localhost:${server.address().port}`, req.headers.host)
@@ -209,7 +209,7 @@ test('trailers dispatch get', async (t) => {
 test('dispatch onHeaders error', async (t) => {
   const p = tspl(t, { plan: 1 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   t.after(closeServerAsPromise(server))
@@ -246,7 +246,7 @@ test('dispatch onHeaders error', async (t) => {
 test('dispatch onComplete error', async (t) => {
   const p = tspl(t, { plan: 2 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   t.after(closeServerAsPromise(server))
@@ -283,7 +283,7 @@ test('dispatch onComplete error', async (t) => {
 test('dispatch onData error', async (t) => {
   const p = tspl(t, { plan: 2 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ad')
   })
   t.after(closeServerAsPromise(server))
@@ -320,7 +320,7 @@ test('dispatch onData error', async (t) => {
 test('dispatch onConnect error', async (t) => {
   const p = tspl(t, { plan: 1 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ad')
   })
   t.after(closeServerAsPromise(server))
@@ -358,7 +358,7 @@ test('dispatch onConnect error', async (t) => {
 test('connect call onUpgrade once', async (t) => {
   const p = tspl(t, { plan: 2 })
 
-  const server = http.createServer((c) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (c) => {
     p.ok(0)
   })
   server.on('connect', (req, socket, firstBodyChunk) => {
@@ -422,7 +422,7 @@ test('connect call onUpgrade once', async (t) => {
 test('dispatch onHeaders missing', async (t) => {
   const p = tspl(t, { plan: 1 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ad')
   })
   t.after(closeServerAsPromise(server))
@@ -455,7 +455,7 @@ test('dispatch onHeaders missing', async (t) => {
 test('dispatch onData missing', async (t) => {
   const p = tspl(t, { plan: 1 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ad')
   })
   t.after(closeServerAsPromise(server))
@@ -488,7 +488,7 @@ test('dispatch onData missing', async (t) => {
 test('dispatch onComplete missing', async (t) => {
   const p = tspl(t, { plan: 1 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ad')
   })
   t.after(closeServerAsPromise(server))
@@ -521,7 +521,7 @@ test('dispatch onComplete missing', async (t) => {
 test('dispatch onError missing', async (t) => {
   const p = tspl(t, { plan: 1 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ad')
   })
   t.after(closeServerAsPromise(server))
@@ -558,7 +558,7 @@ test('dispatch onError missing', async (t) => {
 test('dispatch CONNECT onUpgrade missing', async (t) => {
   const p = tspl(t, { plan: 2 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ad')
   })
   t.after(closeServerAsPromise(server))
@@ -589,7 +589,7 @@ test('dispatch CONNECT onUpgrade missing', async (t) => {
 test('dispatch upgrade onUpgrade missing', async (t) => {
   const p = tspl(t, { plan: 2 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ad')
   })
   t.after(closeServerAsPromise(server))
@@ -620,7 +620,7 @@ test('dispatch upgrade onUpgrade missing', async (t) => {
 test('dispatch pool onError missing', async (t) => {
   const p = tspl(t, { plan: 2 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ad')
   })
   t.after(closeServerAsPromise(server))
@@ -647,7 +647,7 @@ test('dispatch pool onError missing', async (t) => {
 
 test('dispatch onBodySent not a function', async (t) => {
   const p = tspl(t, { plan: 2 })
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ad')
   })
   t.after(closeServerAsPromise(server))
@@ -677,7 +677,7 @@ test('dispatch onBodySent not a function', async (t) => {
 test('dispatch onBodySent buffer', async (t) => {
   const p = tspl(t, { plan: 3 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ad')
   })
   t.after(closeServerAsPromise(server))
@@ -714,7 +714,7 @@ test('dispatch onBodySent buffer', async (t) => {
 
 test('dispatch onBodySent stream', async (t) => {
   const p = tspl(t, { plan: 8 })
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ad')
   })
   t.after(closeServerAsPromise(server))
@@ -756,7 +756,7 @@ test('dispatch onBodySent stream', async (t) => {
 })
 
 test('dispatch onBodySent async-iterable', (t, done) => {
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ad')
   })
   t.after(closeServerAsPromise(server))
@@ -792,7 +792,7 @@ test('dispatch onBodySent async-iterable', (t, done) => {
 })
 
 test('dispatch onBodySent throws error', (t, done) => {
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ended')
   })
   t.after(closeServerAsPromise(server))
@@ -823,7 +823,7 @@ test('dispatch onBodySent throws error', (t, done) => {
 })
 
 test('dispatches in expected order', async (t) => {
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('ended')
   })
   t.after(closeServerAsPromise(server))
@@ -929,7 +929,7 @@ test('dispatches in expected order for http2', async (t) => {
 
 test('Issue#3065 - fix bad destroy handling', async (t) => {
   const p = tspl(t, { plan: 4 })
-  const server = https.createServer(pem, (req, res) => {
+  const server = https.createServer({ ...pem, joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200, { 'content-type': 'text/plain' })
     res.end('ended')
   })
diff --git a/test/node-test/client-errors.js b/test/node-test/client-errors.js
index 39f04678dc6..83a7e8a5064 100644
--- a/test/node-test/client-errors.js
+++ b/test/node-test/client-errors.js
@@ -20,7 +20,7 @@ class IteratorError extends Error {}
 test('GET errors and reconnect with pipelining 1', async (t) => {
   const p = tspl(t, { plan: 9 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
 
   server.once('request', (req, res) => {
     // first request received, destroying
@@ -65,7 +65,7 @@ test('GET errors and reconnect with pipelining 1', async (t) => {
 })
 
 test('GET errors and reconnect with pipelining 3', async (t) => {
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const requestsThatWillError = 3
   let requests = 0
 
@@ -126,7 +126,7 @@ function errorAndPipelining (type) {
   test(`POST with a ${type} that errors and pipelining 1 should reconnect`, async (t) => {
     const p = tspl(t, { plan: 12 })
 
-    const server = createServer()
+    const server = createServer({ joinDuplicateHeaders: true })
     server.once('request', (req, res) => {
       p.strictEqual('/', req.url)
       p.strictEqual('POST', req.method)
@@ -201,7 +201,7 @@ function errorAndChunkedEncodingPipelining (type) {
   test(`POST with chunked encoding, ${type} body that errors and pipelining 1 should reconnect`, async (t) => {
     const p = tspl(t, { plan: 12 })
 
-    const server = createServer()
+    const server = createServer({ joinDuplicateHeaders: true })
     server.once('request', (req, res) => {
       p.strictEqual('/', req.url)
       p.strictEqual('POST', req.method)
@@ -600,7 +600,7 @@ test('invalid options throws', (t, done) => {
 test('POST which fails should error response', async (t) => {
   const p = tspl(t, { plan: 6 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.on('request', (req, res) => {
     req.once('data', () => {
       res.destroy()
@@ -693,7 +693,7 @@ test('client destroy cleanup', async (t) => {
 
   const _err = new Error('kaboom')
   let client
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.once('request', (req, res) => {
     req.once('data', () => {
       client.destroy(_err, (err) => {
@@ -728,7 +728,7 @@ test('client destroy cleanup', async (t) => {
 test('throwing async-iterator causes error', async (t) => {
   const p = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end(Buffer.alloc(4 + 1, 'a'))
   })
   t.after(closeServerAsPromise(server))
@@ -757,7 +757,7 @@ test('client async-iterator destroy cleanup', async (t) => {
 
   const _err = new Error('kaboom')
   let client
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.once('request', (req, res) => {
     req.once('data', () => {
       client.destroy(_err, (err) => {
@@ -788,7 +788,7 @@ test('client async-iterator destroy cleanup', async (t) => {
 test('GET errors body', async (t) => {
   const p = tspl(t, { plan: 2 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.once('request', (req, res) => {
     res.write('asd')
     setTimeout(() => {
@@ -816,7 +816,7 @@ test('GET errors body', async (t) => {
 test('validate request body', async (t) => {
   const p = tspl(t, { plan: 6 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   t.after(closeServerAsPromise(server))
@@ -884,7 +884,7 @@ function socketFailWrite (type) {
   test(`socket fail while writing ${type} request body`, async (t) => {
     const p = tspl(t, { plan: 2 })
 
-    const server = createServer()
+    const server = createServer({ joinDuplicateHeaders: true })
     server.once('request', (req, res) => {
     })
     t.after(closeServerAsPromise(server))
@@ -924,7 +924,7 @@ function socketFailEndWrite (type) {
   test(`socket fail while ending ${type} request body`, async (t) => {
     const p = tspl(t, { plan: 3 })
 
-    const server = createServer()
+    const server = createServer({ joinDuplicateHeaders: true })
     server.once('request', (req, res) => {
       res.end()
     })
@@ -971,7 +971,7 @@ socketFailEndWrite(consts.ASYNC_ITERATOR)
 test('queued request should not fail on socket destroy', async (t) => {
   const p = tspl(t, { plan: 4 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.on('request', (req, res) => {
     res.end()
   })
@@ -1010,7 +1010,7 @@ test('queued request should not fail on socket destroy', async (t) => {
 test('queued request should fail on client destroy', async (t) => {
   const p = tspl(t, { plan: 6 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.on('request', (req, res) => {
     res.end()
   })
@@ -1054,7 +1054,7 @@ test('queued request should fail on client destroy', async (t) => {
 test('retry idempotent inflight', async (t) => {
   const p = tspl(t, { plan: 3 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.on('request', (req, res) => {
     res.end()
   })
@@ -1161,7 +1161,7 @@ test('default port for http and https', async (t) => {
 test('CONNECT throws in next tick', async (t) => {
   const p = tspl(t, { plan: 3 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.on('request', (req, res) => {
     res.end()
   })
@@ -1225,7 +1225,7 @@ test('invalid signal', async (t) => {
 test('invalid body chunk does not crash', async (t) => {
   const p = tspl(t, { plan: 1 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.on('request', (req, res) => {
     res.end()
   })
@@ -1269,7 +1269,7 @@ test('socket errors', async (t) => {
 
 test('headers overflow', (t, done) => {
   const p = tspl(t, { plan: 2 })
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.on('request', (req, res) => {
     res.writeHead(200, {
       'x-test-1': '1',
@@ -1296,7 +1296,7 @@ test('headers overflow', (t, done) => {
 test('SocketError should expose socket details (net)', async (t) => {
   const p = tspl(t, { plan: 8 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
 
   server.once('request', (req, res) => {
     res.destroy()
@@ -1330,7 +1330,7 @@ test('SocketError should expose socket details (net)', async (t) => {
 test('SocketError should expose socket details (tls)', async (t) => {
   const p = tspl(t, { plan: 8 })
 
-  const server = https.createServer(pem)
+  const server = https.createServer({ ...pem, joinDuplicateHeaders: true })
 
   server.once('request', (req, res) => {
     res.destroy()
@@ -1369,7 +1369,7 @@ test('SocketError should expose socket details (tls)', async (t) => {
 test('parser error', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = net.createServer()
+  const server = net.createServer({ joinDuplicateHeaders: true })
   server.once('connection', (socket) => {
     socket.write('asd\n\r213123')
   })
diff --git a/test/node-test/diagnostics-channel/error.js b/test/node-test/diagnostics-channel/error.js
index ce6e0978146..1a62055d533 100644
--- a/test/node-test/diagnostics-channel/error.js
+++ b/test/node-test/diagnostics-channel/error.js
@@ -8,7 +8,7 @@ const { createServer } = require('node:http')
 
 test('Diagnostics channel - error', (t) => {
   const assert = tspl(t, { plan: 3 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.destroy()
   })
   after(server.close.bind(server))
diff --git a/test/node-test/diagnostics-channel/get.js b/test/node-test/diagnostics-channel/get.js
index 397dfa3bc5f..20b2672c22a 100644
--- a/test/node-test/diagnostics-channel/get.js
+++ b/test/node-test/diagnostics-channel/get.js
@@ -8,7 +8,7 @@ const { createServer } = require('node:http')
 
 test('Diagnostics channel - get', (t) => {
   const assert = tspl(t, { plan: 32 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('Content-Type', 'text/plain')
     res.setHeader('trailer', 'foo')
     res.write('hello')
diff --git a/test/node-test/diagnostics-channel/post-stream.js b/test/node-test/diagnostics-channel/post-stream.js
index 881873a7c1c..27cafe2af56 100644
--- a/test/node-test/diagnostics-channel/post-stream.js
+++ b/test/node-test/diagnostics-channel/post-stream.js
@@ -9,7 +9,7 @@ const { createServer } = require('node:http')
 
 test('Diagnostics channel - post stream', (t) => {
   const assert = tspl(t, { plan: 33 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.resume()
     res.setHeader('Content-Type', 'text/plain')
     res.setHeader('trailer', 'foo')
diff --git a/test/node-test/diagnostics-channel/post.js b/test/node-test/diagnostics-channel/post.js
index 1408ffbf023..d18de2a567a 100644
--- a/test/node-test/diagnostics-channel/post.js
+++ b/test/node-test/diagnostics-channel/post.js
@@ -8,7 +8,7 @@ const { createServer } = require('node:http')
 
 test('Diagnostics channel - post', (t) => {
   const assert = tspl(t, { plan: 33 })
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     req.resume()
     res.setHeader('Content-Type', 'text/plain')
     res.setHeader('trailer', 'foo')
diff --git a/test/node-test/large-body.js b/test/node-test/large-body.js
index 8707fe04d03..615e7319865 100644
--- a/test/node-test/large-body.js
+++ b/test/node-test/large-body.js
@@ -8,7 +8,7 @@ const { strictEqual } = require('node:assert')
 test('socket should not be reused unless body is consumed', async (t) => {
   const LARGE_BODY = 'x'.repeat(10000000)
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (req.url === '/foo') {
       res.end(LARGE_BODY)
       return
diff --git a/test/node-test/unix.js b/test/node-test/unix.js
index cdeda6d1b3d..6ea46146583 100644
--- a/test/node-test/unix.js
+++ b/test/node-test/unix.js
@@ -14,7 +14,7 @@ test('http unix get', { skip }, async (t) => {
   let client
   const p = tspl(t, { plan: 7 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     p.equal('/', req.url)
     p.equal('GET', req.method)
     p.equal('localhost', req.headers.host)
@@ -63,7 +63,7 @@ test('http unix get pool', { skip }, async (t) => {
   let client
   const p = tspl(t, { plan: 7 })
 
-  const server = http.createServer((req, res) => {
+  const server = http.createServer({ joinDuplicateHeaders: true }, (req, res) => {
     p.equal('/', req.url)
     p.equal('GET', req.method)
     p.equal('localhost', req.headers.host)
@@ -112,7 +112,7 @@ test('https get with tls opts', { skip }, async (t) => {
   let client
   const p = tspl(t, { plan: 6 })
 
-  const server = https.createServer(pem, (req, res) => {
+  const server = https.createServer({ ...pem, joinDuplicateHeaders: true }, (req, res) => {
     p.equal('/', req.url)
     p.equal('GET', req.method)
     res.setHeader('content-type', 'text/plain')
diff --git a/test/node-test/validations.js b/test/node-test/validations.js
index f23f910ed21..5f25b834dcb 100644
--- a/test/node-test/validations.js
+++ b/test/node-test/validations.js
@@ -9,7 +9,7 @@ test('validations', async t => {
   let client
   const p = tspl(t, { plan: 10 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.setHeader('content-type', 'text/plain')
     res.end('hello')
     p.fail('server should never be called')
diff --git a/test/pipeline-pipelining.js b/test/pipeline-pipelining.js
index 27ea1bb21ed..bb4cf22e84f 100644
--- a/test/pipeline-pipelining.js
+++ b/test/pipeline-pipelining.js
@@ -10,7 +10,7 @@ const { kBusy, kPending, kRunning } = require('../lib/core/symbols')
 test('pipeline pipelining', async (t) => {
   t = tspl(t, { plan: 10 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.deepStrictEqual(req.headers['transfer-encoding'], undefined)
     res.end()
   })
@@ -54,7 +54,7 @@ test('pipeline pipelining retry', async (t) => {
   t = tspl(t, { plan: 13 })
 
   let count = 0
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (count++ === 0) {
       res.destroy()
     } else {
diff --git a/test/pool-connection-error-memory-leak.js b/test/pool-connection-error-memory-leak.js
index fc9c7deb4cf..7ecaa601c4e 100644
--- a/test/pool-connection-error-memory-leak.js
+++ b/test/pool-connection-error-memory-leak.js
@@ -102,7 +102,7 @@ test('Pool client count does not grow on repeated connection errors', async (t)
 // This test specifically verifies the fix in pool-base.js for connectionError event
 test('Pool clients are removed on connectionError event', async (t) => {
   // Create a server we'll use to track connection events
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200, { 'Content-Type': 'text/plain' })
     res.end('ok')
   })
diff --git a/test/pool.js b/test/pool.js
index b75cd530d43..2b847af70d3 100644
--- a/test/pool.js
+++ b/test/pool.js
@@ -79,7 +79,7 @@ test('connect/disconnect event(s)', async (t) => {
 
   t = tspl(t, { plan: clients * 6 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200, {
       Connection: 'keep-alive',
       'Keep-Alive': 'timeout=1s'
@@ -122,7 +122,7 @@ test('connect/disconnect event(s)', async (t) => {
 test('basic get', async (t) => {
   t = tspl(t, { plan: 14 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     res.setHeader('content-type', 'text/plain')
@@ -170,7 +170,7 @@ test('basic get', async (t) => {
 test('URL as arg', async (t) => {
   t = tspl(t, { plan: 9 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     res.setHeader('content-type', 'text/plain')
@@ -214,7 +214,7 @@ test('URL as arg', async (t) => {
 test('basic get error async/await', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.destroy()
   })
   after(() => server.close())
@@ -241,7 +241,7 @@ test('basic get error async/await', async (t) => {
 test('basic get with async/await', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     res.setHeader('content-type', 'text/plain')
@@ -267,7 +267,7 @@ test('basic get with async/await', async (t) => {
 test('stream get async/await', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     res.setHeader('content-type', 'text/plain')
@@ -291,7 +291,7 @@ test('stream get async/await', async (t) => {
 test('stream get error async/await', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.destroy()
   })
   after(() => server.close())
@@ -314,7 +314,7 @@ test('stream get error async/await', async (t) => {
 test('pipeline get', async (t) => {
   t = tspl(t, { plan: 5 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     res.setHeader('content-type', 'text/plain')
@@ -425,7 +425,7 @@ test('backpressure algorithm', async (t) => {
 test('busy', async (t) => {
   t = tspl(t, { plan: 8 * 16 + 2 + 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     res.setHeader('content-type', 'text/plain')
@@ -488,7 +488,7 @@ test('invalid pool dispatch options', async (t) => {
 test('pool upgrade promise', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = net.createServer((c) => {
+  const server = net.createServer({ joinDuplicateHeaders: true }, (c) => {
     c.on('data', (d) => {
       c.write('HTTP/1.1 101\r\n')
       c.write('hello: world\r\n')
@@ -537,7 +537,7 @@ test('pool upgrade promise', async (t) => {
 test('pool connect', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((c) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (c) => {
     t.fail()
   })
   server.on('connect', (req, socket, firstBodyChunk) => {
@@ -581,7 +581,7 @@ test('pool connect', async (t) => {
 test('pool dispatch', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -631,7 +631,7 @@ test('pool pipeline args validation', async (t) => {
 test('300 requests succeed', async (t) => {
   t = tspl(t, { plan: 300 * 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -663,7 +663,7 @@ test('300 requests succeed', async (t) => {
 test('pool connect error', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((c) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (c) => {
     t.fail()
   })
   server.on('connect', (req, socket, firstBodyChunk) => {
@@ -690,7 +690,7 @@ test('pool connect error', async (t) => {
 test('pool upgrade error', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = net.createServer((c) => {
+  const server = net.createServer({ joinDuplicateHeaders: true }, (c) => {
     c.on('data', (d) => {
       c.write('HTTP/1.1 101\r\n')
       c.write('hello: world\r\n')
@@ -726,7 +726,7 @@ test('pool upgrade error', async (t) => {
 test('pool dispatch error', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -784,7 +784,7 @@ test('pool dispatch error', async (t) => {
 test('pool request abort in queue', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -831,7 +831,7 @@ test('pool request abort in queue', async (t) => {
 test('pool stream abort in queue', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -878,7 +878,7 @@ test('pool stream abort in queue', async (t) => {
 test('pool pipeline abort in queue', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -925,7 +925,7 @@ test('pool pipeline abort in queue', async (t) => {
 test('pool stream constructor error destroy body', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -981,7 +981,7 @@ test('pool stream constructor error destroy body', async (t) => {
 test('pool request constructor error destroy body', async (t) => {
   t = tspl(t, { plan: 4 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -1033,7 +1033,7 @@ test('pool request constructor error destroy body', async (t) => {
 test('pool close waits for all requests', async (t) => {
   t = tspl(t, { plan: 5 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -1081,7 +1081,7 @@ test('pool close waits for all requests', async (t) => {
 test('pool destroyed', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -1108,7 +1108,7 @@ test('pool destroyed', async (t) => {
 test('pool destroy fails queued requests', async (t) => {
   t = tspl(t, { plan: 6 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
diff --git a/test/promises.js b/test/promises.js
index 1eddfc216f6..3aaf909cd71 100644
--- a/test/promises.js
+++ b/test/promises.js
@@ -10,7 +10,7 @@ const { wrapWithAsyncIterable } = require('./utils/async-iterators')
 test('basic get, async await support', async (t) => {
   t = tspl(t, { plan: 5 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     res.setHeader('content-type', 'text/plain')
@@ -63,7 +63,7 @@ test('basic POST with string, async await support', async (t) => {
 
   const expected = readFileSync(__filename, 'utf8')
 
-  const server = createServer(postServer(t, expected))
+  const server = createServer({ joinDuplicateHeaders: true }, postServer(t, expected))
   after(() => server.close())
 
   server.listen(0, async () => {
@@ -93,7 +93,7 @@ test('basic POST with Buffer, async await support', async (t) => {
 
   const expected = readFileSync(__filename)
 
-  const server = createServer(postServer(t, expected.toString()))
+  const server = createServer({ joinDuplicateHeaders: true }, postServer(t, expected.toString()))
   after(() => server.close())
 
   server.listen(0, async () => {
@@ -123,7 +123,7 @@ test('basic POST with stream, async await support', async (t) => {
 
   const expected = readFileSync(__filename, 'utf8')
 
-  const server = createServer(postServer(t, expected))
+  const server = createServer({ joinDuplicateHeaders: true }, postServer(t, expected))
   after(() => server.close())
 
   server.listen(0, async () => {
@@ -160,7 +160,7 @@ test('basic POST with async-iterator, async await support', async (t) => {
 
   const expected = readFileSync(__filename, 'utf8')
 
-  const server = createServer(postServer(t, expected))
+  const server = createServer({ joinDuplicateHeaders: true }, postServer(t, expected))
   after(() => server.close())
 
   server.listen(0, async () => {
@@ -202,7 +202,7 @@ test('20 times GET with pipelining 10, async await support', async (t) => {
 
   let count = 0
   let countGreaterThanOne = false
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     count++
     await sleep(10)
     countGreaterThanOne = countGreaterThanOne || count > 1
@@ -263,7 +263,7 @@ async function makeRequestAndExpectUrl (client, i, t) {
 test('pool, async await support', async (t) => {
   t = tspl(t, { plan: 5 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     res.setHeader('content-type', 'text/plain')
diff --git a/test/proxy-agent.js b/test/proxy-agent.js
index 6dcdfee450c..a412b4adebf 100644
--- a/test/proxy-agent.js
+++ b/test/proxy-agent.js
@@ -875,7 +875,7 @@ test('ProxyAgent keeps customized host in request headers - #3019', async (t) =>
 
 function buildServer () {
   return new Promise((resolve) => {
-    const server = createServer()
+    const server = createServer({ joinDuplicateHeaders: true })
     server.listen(0, () => resolve(server))
   })
 }
@@ -886,7 +886,8 @@ function buildSSLServer () {
       certs.root.crt
     ],
     key: certs.server.key,
-    cert: certs.server.crt
+    cert: certs.server.crt,
+    joinDuplicateHeaders: true
   }
   return new Promise((resolve) => {
     const server = https.createServer(serverOptions)
@@ -898,7 +899,7 @@ function buildProxy (listener) {
   return new Promise((resolve) => {
     const server = listener
       ? createProxy(createServer(listener))
-      : createProxy(createServer())
+      : createProxy(createServer({ joinDuplicateHeaders: true }))
     server.listen(0, () => resolve(server))
   })
 }
@@ -909,7 +910,8 @@ function buildSSLProxy () {
       certs.root.crt
     ],
     key: certs.proxy.key,
-    cert: certs.proxy.crt
+    cert: certs.proxy.crt,
+    joinDuplicateHeaders: true
   }
 
   return new Promise((resolve) => {
diff --git a/test/proxy.js b/test/proxy.js
index 30d1290a6ff..86cae7bfc93 100644
--- a/test/proxy.js
+++ b/test/proxy.js
@@ -154,14 +154,14 @@ test('connect through proxy (with pool)', async (t) => {
 
 function buildServer () {
   return new Promise((resolve, reject) => {
-    const server = createServer()
+    const server = createServer({ joinDuplicateHeaders: true })
     server.listen(0, () => resolve(server))
   })
 }
 
 function buildProxy () {
   return new Promise((resolve, reject) => {
-    const server = createProxy(createServer())
+    const server = createProxy(createServer({ joinDuplicateHeaders: true }))
     server.listen(0, () => resolve(server))
   })
 }
diff --git a/test/request-crlf.js b/test/request-crlf.js
index 4e6921e2fb0..38d5e58b61e 100644
--- a/test/request-crlf.js
+++ b/test/request-crlf.js
@@ -9,7 +9,7 @@ const { once } = require('node:events')
 test('should validate content-type CRLF Injection', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.fail('should not receive any request')
     res.statusCode = 200
     res.end('hello')
diff --git a/test/request-signal.js b/test/request-signal.js
index fd4d2f885a5..73df93897a8 100644
--- a/test/request-signal.js
+++ b/test/request-signal.js
@@ -8,7 +8,7 @@ const { request } = require('..')
 test('pre abort signal w/ reason', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -29,7 +29,7 @@ test('pre abort signal w/ reason', async (t) => {
 test('post abort signal', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
@@ -53,7 +53,7 @@ test('post abort signal', async (t) => {
 test('post abort signal w/ reason', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('asd')
   })
   after(() => server.close())
diff --git a/test/request-timeout.js b/test/request-timeout.js
index 19f602fff8b..d56d2a9c14a 100644
--- a/test/request-timeout.js
+++ b/test/request-timeout.js
@@ -28,7 +28,7 @@ beforeEach(() => {
 test('request timeout', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       res.end('hello')
     }, 2000)
@@ -50,7 +50,7 @@ test('request timeout', async (t) => {
 test('request timeout with readable body', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
   })
   after(() => server.close())
 
@@ -80,7 +80,7 @@ test('body timeout', async (t) => {
   })
   after(() => clock.uninstall())
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write('hello')
   })
   after(() => server.close())
@@ -115,7 +115,7 @@ test('overridden request timeout', async (t) => {
   })
   after(() => clock.uninstall())
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       res.end('hello')
     }, 100)
@@ -148,7 +148,7 @@ test('overridden body timeout', async (t) => {
   })
   after(() => clock.uninstall())
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.write('hello')
   })
   after(() => server.close())
@@ -183,7 +183,7 @@ test('With EE signal', async (t) => {
   })
   after(() => clock.uninstall())
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       res.end('hello')
     }, 100)
@@ -219,7 +219,7 @@ test('With abort-controller signal', async (t) => {
   })
   after(() => clock.uninstall())
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       res.end('hello')
     }, 100)
@@ -256,7 +256,7 @@ test('Abort before timeout (EE)', async (t) => {
   after(() => clock.uninstall())
 
   const ee = new EventEmitter()
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       res.end('hello')
     }, 100)
@@ -292,7 +292,7 @@ test('Abort before timeout (abort-controller)', async (t) => {
   after(() => clock.uninstall())
 
   const abortController = new AbortController()
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       res.end('hello')
     }, 100)
@@ -327,7 +327,7 @@ test('Timeout with pipelining', async (t) => {
   })
   after(() => clock.uninstall())
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       res.end('hello')
     }, 100)
@@ -368,7 +368,7 @@ test('Global option', async (t) => {
   })
   after(() => clock.uninstall())
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       res.end('hello')
     }, 100)
@@ -403,7 +403,7 @@ test('Request options overrides global option', async (t) => {
   })
   after(() => clock.uninstall())
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       res.end('hello')
     }, 100)
@@ -432,7 +432,7 @@ test('Request options overrides global option', async (t) => {
 test('client.destroy should cancel the timeout', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end('hello')
   })
   after(() => server.close())
@@ -463,7 +463,7 @@ test('client.close should wait for the timeout', async (t) => {
   })
   after(() => clock.uninstall())
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
   })
   after(() => server.close())
 
@@ -543,7 +543,7 @@ test('Disable request timeout', async (t) => {
   })
   after(() => clock.uninstall())
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       res.end('hello')
     }, 32e3)
@@ -586,7 +586,7 @@ test('Disable request timeout for a single request', async (t) => {
   })
   after(() => clock.uninstall())
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       res.end('hello')
     }, 32e3)
@@ -629,7 +629,7 @@ test('stream timeout', async (t) => {
   })
   after(() => clock.uninstall())
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       res.end('hello')
     }, 301e3)
@@ -665,7 +665,7 @@ test('stream custom timeout', async (t) => {
   })
   after(() => clock.uninstall())
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       res.end('hello')
     }, 31e3)
@@ -703,7 +703,7 @@ test('pipeline timeout', async (t) => {
   })
   after(() => clock.uninstall())
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       req.pipe(res)
     }, 301e3)
@@ -758,7 +758,7 @@ test('pipeline timeout', async (t) => {
   })
   after(() => clock.uninstall())
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     setTimeout(() => {
       req.pipe(res)
     }, 31e3)
@@ -815,7 +815,7 @@ test('client.close should not deadlock', async (t) => {
   })
   after(() => clock.uninstall())
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
   })
   after(() => server.close())
 
diff --git a/test/request-timeout2.js b/test/request-timeout2.js
index a294b2fb9c1..48293f81e95 100644
--- a/test/request-timeout2.js
+++ b/test/request-timeout2.js
@@ -10,7 +10,7 @@ const { Readable } = require('node:stream')
 test('request timeout with slow readable body', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer(async (req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
     let str = ''
     for await (const x of req) {
       str += x
diff --git a/test/request.js b/test/request.js
index 38f8cdc1aae..059a321f2ba 100644
--- a/test/request.js
+++ b/test/request.js
@@ -8,13 +8,13 @@ const { request, errors } = require('..')
 test('no-slash/one-slash pathname should be included in req.path', async (t) => {
   t = tspl(t, { plan: 24 })
 
-  const pathServer = createServer((req, res) => {
+  const pathServer = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.fail('it shouldn\'t be called')
     res.statusCode = 200
     res.end('hello')
   })
 
-  const requestedServer = createServer((req, res) => {
+  const requestedServer = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(`/localhost:${pathServer.address().port}`, req.url)
     t.strictEqual('GET', req.method)
     t.strictEqual(`localhost:${requestedServer.address().port}`, req.headers.host)
@@ -72,13 +72,13 @@ test('no-slash/one-slash pathname should be included in req.path', async (t) =>
 test('protocol-relative URL as pathname should be included in req.path', async (t) => {
   t = tspl(t, { plan: 12 })
 
-  const pathServer = createServer((req, res) => {
+  const pathServer = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.fail('it shouldn\'t be called')
     res.statusCode = 200
     res.end('hello')
   })
 
-  const requestedServer = createServer((req, res) => {
+  const requestedServer = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(`//localhost:${pathServer.address().port}`, req.url)
     t.strictEqual('GET', req.method)
     t.strictEqual(`localhost:${requestedServer.address().port}`, req.headers.host)
@@ -119,13 +119,13 @@ test('protocol-relative URL as pathname should be included in req.path', async (
 test('Absolute URL as pathname should be included in req.path', async (t) => {
   t = tspl(t, { plan: 12 })
 
-  const pathServer = createServer((req, res) => {
+  const pathServer = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.fail('it shouldn\'t be called')
     res.statusCode = 200
     res.end('hello')
   })
 
-  const requestedServer = createServer((req, res) => {
+  const requestedServer = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual(`/http://localhost:${pathServer.address().port}`, req.url)
     t.strictEqual('GET', req.method)
     t.strictEqual(`localhost:${requestedServer.address().port}`, req.headers.host)
@@ -193,7 +193,7 @@ describe('DispatchOptions#reset', () => {
   test('Should include "connection:close" if reset true', async t => {
     t = tspl(t, { plan: 3 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       t.strictEqual('GET', req.method)
       t.strictEqual(`localhost:${server.address().port}`, req.headers.host)
       t.strictEqual(req.headers.connection, 'close')
@@ -220,7 +220,7 @@ describe('DispatchOptions#reset', () => {
   test('Should include "connection:keep-alive" if reset false', async t => {
     t = tspl(t, { plan: 3 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       t.strictEqual('GET', req.method)
       t.strictEqual(`localhost:${server.address().port}`, req.headers.host)
       t.strictEqual(req.headers.connection, 'keep-alive')
@@ -247,7 +247,7 @@ describe('DispatchOptions#reset', () => {
   test('Should react to manual set of "connection:close" header', async t => {
     t = tspl(t, { plan: 3 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       t.strictEqual('GET', req.method)
       t.strictEqual(`localhost:${server.address().port}`, req.headers.host)
       t.strictEqual(req.headers.connection, 'close')
@@ -278,7 +278,7 @@ describe('Should include headers from iterable objects', scope => {
   test('Should include headers built with Headers global object', { skip: !globalThis.Headers }, async t => {
     t = tspl(t, { plan: 3 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       t.strictEqual('GET', req.method)
       t.strictEqual(`localhost:${server.address().port}`, req.headers.host)
       t.strictEqual(req.headers.hello, 'world')
@@ -309,7 +309,7 @@ describe('Should include headers from iterable objects', scope => {
   test('Should include headers built with Map', async t => {
     t = tspl(t, { plan: 3 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       t.strictEqual('GET', req.method)
       t.strictEqual(`localhost:${server.address().port}`, req.headers.host)
       t.strictEqual(req.headers.hello, 'world')
@@ -340,7 +340,7 @@ describe('Should include headers from iterable objects', scope => {
   test('Should include headers built with custom iterable object', async t => {
     t = tspl(t, { plan: 3 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       t.strictEqual('GET', req.method)
       t.strictEqual(`localhost:${server.address().port}`, req.headers.host)
       t.strictEqual(req.headers.hello, 'world')
@@ -374,7 +374,7 @@ describe('Should include headers from iterable objects', scope => {
   test('Should throw error if headers iterable object does not yield key-value pairs', async t => {
     t = tspl(t, { plan: 2 })
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.end('hello')
     })
 
diff --git a/test/retry-agent.js b/test/retry-agent.js
index 0e5e252d954..f4edaa5f4a0 100644
--- a/test/retry-agent.js
+++ b/test/retry-agent.js
@@ -10,7 +10,7 @@ test('Should retry status code', async t => {
   t = tspl(t, { plan: 2 })
 
   let counter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const opts = {
     maxRetries: 5,
     timeout: 1,
diff --git a/test/retry-handler.js b/test/retry-handler.js
index 85eea6b0597..3a42fce3a7a 100644
--- a/test/retry-handler.js
+++ b/test/retry-handler.js
@@ -14,7 +14,7 @@ test('Should retry status code', async t => {
 
   let counter = 0
   const chunks = []
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const dispatchOptions = {
     retryOptions: {
       retry: (err, { state, opts }, done) => {
@@ -109,7 +109,7 @@ test('Should account for network and response errors', async t => {
 
   let counter = 0
   const chunks = []
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const dispatchOptions = {
     retryOptions: {
       retry: (err, { state, opts }, done) => {
@@ -201,7 +201,7 @@ test('Should account for network and response errors', async t => {
 
 test('Issue #3288 - request with body (asynciterable)', async t => {
   t = tspl(t, { plan: 4 })
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const dispatchOptions = {
     method: 'POST',
     path: '/',
@@ -268,7 +268,7 @@ test('Should use retry-after header for retries', async t => {
 
   let counter = 0
   const chunks = []
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   let checkpoint
   const dispatchOptions = {
     method: 'PUT',
@@ -351,7 +351,7 @@ test('Should use retry-after header for retries (date)', async t => {
 
   let counter = 0
   const chunks = []
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   let checkpoint
   const dispatchOptions = {
     method: 'PUT',
@@ -436,7 +436,7 @@ test('Should retry with defaults', async t => {
 
   let counter = 0
   const chunks = []
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const dispatchOptions = {
     method: 'GET',
     path: '/',
@@ -521,7 +521,7 @@ test('Should handle 206 partial content', async t => {
 
   // Took from: https://github.com/nxtedition/nxt-lib/blob/4b001ebc2f22cf735a398f35ff800dd553fe5933/test/undici/retry.js#L47
   let x = 0
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (x === 0) {
       t.ok(true, 'pass')
       res.setHeader('etag', 'asd')
@@ -617,7 +617,7 @@ test('Should handle 206 partial content - bad-etag', async t => {
 
   // Took from: https://github.com/nxtedition/nxt-lib/blob/4b001ebc2f22cf735a398f35ff800dd553fe5933/test/undici/retry.js#L47
   let x = 0
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (x === 0) {
       t.ok(true, 'pass')
       res.setHeader('etag', 'asd')
@@ -699,7 +699,7 @@ test('Should handle 206 partial content - bad-etag', async t => {
 
 test('retrying a request with a body', async t => {
   let counter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const dispatchOptions = {
     retryOptions: {
       retry: (err, { state, opts }, done) => {
@@ -778,7 +778,7 @@ test('retrying a request with a body', async t => {
 
 test('retrying a request with a body (stream)', async t => {
   let counter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const dispatchOptions = {
     retryOptions: {
       retry: (err, { state, opts }, done) => {
@@ -845,7 +845,7 @@ test('retrying a request with a body (stream)', async t => {
 
 test('retrying a request with a body (buffer)', async t => {
   let counter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const dispatchOptions = {
     retryOptions: {
       retry: (err, { state, opts }, done) => {
@@ -918,7 +918,7 @@ test('retrying a request with a body (buffer)', async t => {
 test('should not error if request is not meant to be retried', async t => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.on('request', (req, res) => {
     res.writeHead(400)
     res.end('Bad request')
@@ -986,7 +986,7 @@ test('Should be able to properly pass the minTimeout to the RetryContext when co
   t = tspl(t, { plan: 2 })
 
   let counter = 0
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   server.on('request', (req, res) => {
     switch (counter) {
       case 0:
@@ -1062,7 +1062,7 @@ test('Issue#2986 - Handle custom 206', async t => {
 
   // Took from: https://github.com/nxtedition/nxt-lib/blob/4b001ebc2f22cf735a398f35ff800dd553fe5933/test/undici/retry.js#L47
   let x = 0
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (x === 0) {
       t.deepStrictEqual(req.headers.range, 'bytes=0-3')
       res.setHeader('etag', 'asd')
@@ -1160,7 +1160,7 @@ test('Issue#3128 - Support if-match', async t => {
 
   // Took from: https://github.com/nxtedition/nxt-lib/blob/4b001ebc2f22cf735a398f35ff800dd553fe5933/test/undici/retry.js#L47
   let x = 0
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (x === 0) {
       t.deepStrictEqual(req.headers.range, 'bytes=0-3')
       res.setHeader('etag', 'asd')
@@ -1260,7 +1260,7 @@ test('Issue#3128 - Should ignore weak etags', async t => {
 
   // Took from: https://github.com/nxtedition/nxt-lib/blob/4b001ebc2f22cf735a398f35ff800dd553fe5933/test/undici/retry.js#L47
   let x = 0
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (x === 0) {
       t.deepStrictEqual(req.headers.range, 'bytes=0-3')
       res.setHeader('etag', 'W/asd')
@@ -1360,7 +1360,7 @@ test('Weak etags are ignored on range-requests', async t => {
 
   // Took from: https://github.com/nxtedition/nxt-lib/blob/4b001ebc2f22cf735a398f35ff800dd553fe5933/test/undici/retry.js#L47
   let x = 0
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (x === 0) {
       t.deepStrictEqual(req.headers.range, 'bytes=0-3')
       res.setHeader('etag', 'W/asd')
@@ -1459,7 +1459,7 @@ test('Should throw RequestRetryError when Content-Range mismatch', async t => {
 
   // Took from: https://github.com/nxtedition/nxt-lib/blob/4b001ebc2f22cf735a398f35ff800dd553fe5933/test/undici/retry.js#L47
   let x = 0
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     if (x === 0) {
       t.ok(true, 'pass')
       res.setHeader('etag', 'asd')
@@ -1553,7 +1553,7 @@ test('Should use retry-after header for retries (date) but date format is wrong'
 
   let counter = 0
   const chunks = []
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   let checkpoint
   const dispatchOptions = {
     method: 'PUT',
diff --git a/test/socket-back-pressure.js b/test/socket-back-pressure.js
index 043e815ed9a..e7150c0a9c5 100644
--- a/test/socket-back-pressure.js
+++ b/test/socket-back-pressure.js
@@ -10,7 +10,7 @@ const { test, after } = require('node:test')
 test('socket back-pressure', async (t) => {
   t = tspl(t, { plan: 3 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   let bytesWritten = 0
 
   const buf = Buffer.allocUnsafe(16384)
diff --git a/test/socket-timeout.js b/test/socket-timeout.js
index 43ee1576615..5095da8f430 100644
--- a/test/socket-timeout.js
+++ b/test/socket-timeout.js
@@ -9,7 +9,7 @@ const FakeTimers = require('@sinonjs/fake-timers')
 test('timeout with pipelining 1', async (t) => {
   t = tspl(t, { plan: 9 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
 
   server.once('request', (req, res) => {
     t.ok(true, 'first request received, we are letting this timeout on the client')
@@ -63,7 +63,7 @@ test('timeout with pipelining 1', async (t) => {
 test('Disable socket timeout', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer()
+  const server = createServer({ joinDuplicateHeaders: true })
   const clock = FakeTimers.install()
   after(clock.uninstall.bind(clock))
 
diff --git a/test/stream-compat.js b/test/stream-compat.js
index 5f219fe42e3..15806a2447a 100644
--- a/test/stream-compat.js
+++ b/test/stream-compat.js
@@ -10,7 +10,7 @@ const EE = require('node:events')
 test('stream body without destroy', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   after(() => server.close())
@@ -41,7 +41,7 @@ test('stream body without destroy', async (t) => {
 test('IncomingMessage', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.end()
   })
   after(() => server.close())
@@ -50,7 +50,7 @@ test('IncomingMessage', async (t) => {
     const proxyClient = new Client(`http://localhost:${server.address().port}`)
     after(() => proxyClient.destroy())
 
-    const proxy = createServer((req, res) => {
+    const proxy = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       proxyClient.request({
         path: '/',
         method: 'PUT',
diff --git a/test/tls-cert-leak.js b/test/tls-cert-leak.js
index 2a579e7a0a6..784b6980f75 100644
--- a/test/tls-cert-leak.js
+++ b/test/tls-cert-leak.js
@@ -23,7 +23,8 @@ test('no memory leak with TLS certificate errors', { timeout: 20000 }, async (t)
   // Create HTTPS server with self-signed certificate
   const serverOptions = {
     key: fs.readFileSync(path.join(__dirname, 'fixtures', 'key.pem')),
-    cert: fs.readFileSync(path.join(__dirname, 'fixtures', 'cert.pem'))
+    cert: fs.readFileSync(path.join(__dirname, 'fixtures', 'cert.pem')),
+    joinDuplicateHeaders: true
   }
 
   // Create a server that always responds with a simple message
diff --git a/test/tls-session-reuse.js b/test/tls-session-reuse.js
index c70578bed1f..2ebd25dbd2a 100644
--- a/test/tls-session-reuse.js
+++ b/test/tls-session-reuse.js
@@ -11,7 +11,8 @@ const { kSocket } = require('../lib/core/symbols')
 
 const options = {
   key: readFileSync(join(__dirname, 'fixtures', 'key.pem'), 'utf8'),
-  cert: readFileSync(join(__dirname, 'fixtures', 'cert.pem'), 'utf8')
+  cert: readFileSync(join(__dirname, 'fixtures', 'cert.pem'), 'utf8'),
+  joinDuplicateHeaders: true
 }
 const ca = readFileSync(join(__dirname, 'fixtures', 'ca.pem'), 'utf8')
 
diff --git a/test/trailers.js b/test/trailers.js
index 8f616e87eba..64a29da7b22 100644
--- a/test/trailers.js
+++ b/test/trailers.js
@@ -8,7 +8,7 @@ const { createServer } = require('node:http')
 test('response trailers missing is OK', async (t) => {
   t = tspl(t, { plan: 1 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200, {
       Trailer: 'content-length'
     })
@@ -33,7 +33,7 @@ test('response trailers missing is OK', async (t) => {
 test('response trailers missing w trailers is OK', async (t) => {
   t = tspl(t, { plan: 2 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     res.writeHead(200, {
       Trailer: 'content-length'
     })
diff --git a/test/utils/esm-wrapper.mjs b/test/utils/esm-wrapper.mjs
index 104190c1afe..76d8c2ae426 100644
--- a/test/utils/esm-wrapper.mjs
+++ b/test/utils/esm-wrapper.mjs
@@ -19,7 +19,7 @@ import {
 test('imported Client works with basic GET', async (t) => {
   t = tspl(t, { plan: 10 })
 
-  const server = createServer((req, res) => {
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
     t.strictEqual('/', req.url)
     t.strictEqual('GET', req.method)
     t.strictEqual(`localhost:${server.address().port}`, req.headers.host)
diff --git a/test/utils/hello-world-server.js b/test/utils/hello-world-server.js
index 520ed3a734d..50dd072dc4f 100644
--- a/test/utils/hello-world-server.js
+++ b/test/utils/hello-world-server.js
@@ -3,7 +3,7 @@
 const { createServer } = require('node:http')
 const hostname = '127.0.0.1'
 
-const server = createServer(async (req, res) => {
+const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
   res.statusCode = 200
   res.setHeader('Content-Type', 'text/plain')
 
diff --git a/test/utils/redirecting-servers.js b/test/utils/redirecting-servers.js
index 011979c3e1e..ad9b6e7180c 100644
--- a/test/utils/redirecting-servers.js
+++ b/test/utils/redirecting-servers.js
@@ -18,7 +18,7 @@ function close (server) {
 
 function startServer (handler) {
   return new Promise(resolve => {
-    const server = createServer(handler)
+    const server = createServer({ joinDuplicateHeaders: true }, handler)
 
     server.listen(0, () => {
       resolve(`localhost:${server.address().port}`)
diff --git a/test/websocket/opening-handshake.js b/test/websocket/opening-handshake.js
index 110a827edc9..1eaeb9fecd4 100644
--- a/test/websocket/opening-handshake.js
+++ b/test/websocket/opening-handshake.js
@@ -8,7 +8,7 @@ const { WebSocket } = require('../..')
 
 test('WebSocket connecting to server that isn\'t a Websocket server', () => {
   return new Promise((resolve, reject) => {
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       assert.equal(req.headers.connection, 'upgrade')
       assert.equal(req.headers.upgrade, 'websocket')
       assert.ok(req.headers['sec-websocket-key'])
@@ -68,7 +68,7 @@ test('Multiple protocols are joined by a comma', () => {
 
 test('Server doesn\'t send Sec-WebSocket-Protocol header when protocols are used', () => {
   return new Promise((resolve, reject) => {
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.statusCode = 101
 
       req.socket.destroy()
@@ -88,7 +88,7 @@ test('Server doesn\'t send Sec-WebSocket-Protocol header when protocols are used
 
 test('Server sends invalid Upgrade header', () => {
   return new Promise((resolve, reject) => {
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.setHeader('Upgrade', 'NotWebSocket')
       res.statusCode = 101
 
@@ -109,7 +109,7 @@ test('Server sends invalid Upgrade header', () => {
 
 test('Server sends invalid Connection header', () => {
   return new Promise((resolve, reject) => {
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.setHeader('Upgrade', 'websocket')
       res.setHeader('Connection', 'downgrade')
       res.statusCode = 101
@@ -131,7 +131,7 @@ test('Server sends invalid Connection header', () => {
 
 test('Server sends invalid Sec-WebSocket-Accept header', () => {
   return new Promise((resolve, reject) => {
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       res.setHeader('Upgrade', 'websocket')
       res.setHeader('Connection', 'upgrade')
       res.setHeader('Sec-WebSocket-Accept', 'abc')
@@ -157,7 +157,7 @@ test('Server sends invalid Sec-WebSocket-Extensions header', () => {
     const uid = '258EAFA5-E914-47DA-95CA-C5AB0DC85B11'
     const { createHash } = require('node:crypto')
 
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       const key = req.headers['sec-websocket-key']
       assert.ok(key)
 
@@ -189,7 +189,7 @@ test('Server sends invalid Sec-WebSocket-Extensions header', () => {
   const { createHash } = require('node:crypto')
 
   return new Promise((resolve, reject) => {
-    const server = createServer((req, res) => {
+    const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
       const key = req.headers['sec-websocket-key']
       assert.ok(key)
 
diff --git a/test/wpt/server/server.mjs b/test/wpt/server/server.mjs
index 2ade5291c6e..a83651d38ac 100644
--- a/test/wpt/server/server.mjs
+++ b/test/wpt/server/server.mjs
@@ -30,7 +30,7 @@ class Stash extends Map {
 
 const stash = new Stash()
 
-const server = createServer(async (req, res) => {
+const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
   const fullUrl = new URL(req.url, `http://localhost:${server.address().port}`)
 
   if (fullUrl.searchParams.has('pipe')) {