fix: Do not fire metadataadded event for duplicated cues (#8617)

Author: tykus160

lib/player.js

@@ -1824,6 +1824,17 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
 
       this.metadataRegionTimeline_ =
           new shaka.media.RegionTimeline(bufferRange);
+      this.metadataRegionTimeline_.addEventListener('regionadd', (event) => {
+        /** @type {shaka.extern.MetadataTimelineRegionInfo} */
+        const region = event['region'];
+        const data = new Map()
+            .set('startTime', region.startTime)
+            .set('endTime', region.endTime)
+            .set('metadataType', region.schemeIdUri)
+            .set('payload', region.payload);
+        this.dispatchEvent(shaka.Player.makeEvent_(
+            shaka.util.FakeEvent.EventName.MetadataAdded, data));
+      });
 
       if (shouldUseSrcEquals) {
         await mutexWrapOperation(async () => {
@@ -3619,13 +3630,6 @@ shaka.Player = class extends shaka.util.FakeEventTarget {
     // JSON stringify produces a good ID in this case.
     region.id = JSON.stringify(region);
     this.metadataRegionTimeline_.addRegion(region);
-    const data = new Map()
-        .set('startTime', region.startTime)
-        .set('endTime', region.endTime)
-        .set('metadataType', region.schemeIdUri)
-        .set('payload', region.payload);
-    this.dispatchEvent(shaka.Player.makeEvent_(
-        shaka.util.FakeEvent.EventName.MetadataAdded, data));
   }
 
   /**