unit test for an extra case, slight formatting fix, minor refactor

Author: SimaTian

src/Build.UnitTests/BackEnd/IntrinsicTask_Tests.cs

@@ -314,6 +314,26 @@ public void ItemKeepDuplicatesFalse()
             Assert.Single(group);
         }
 
+        [Fact]
+        public void ItemKeepDuplicatesFalseTwoDuplicatesAtOnce()
+        {
+            string content = ObjectModelHelpers.CleanupFileContents(@"
+            <Project ToolsVersion='msbuilddefaulttoolsversion' xmlns='msbuildnamespace'>
+            <Target Name='t'>
+                <ItemGroup>
+                    <i1 Include='a1'/>
+                    <i1 Include='a1;a1' KeepDuplicates='false' />
+                </ItemGroup>
+            </Target>
+            </Project>");
+            IntrinsicTask task = CreateIntrinsicTask(content);
+            Lookup lookup = LookupHelpers.CreateEmptyLookup();
+            ExecuteTask(task, lookup);
+
+            var group = lookup.GetItems("i1");
+            Assert.Single(group);
+        }
+
         [Fact]
         public void ItemKeepDuplicatesAsCondition()
         {

src/Build/BackEnd/Components/RequestBuilder/IntrinsicTasks/ItemGroupIntrinsicTask.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
+using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
@@ -215,7 +216,7 @@ private void ExecuteAdd(ProjectItemGroupTaskItemInstance child, ItemBucket bucke
                 FileSystems.Default,
                 LoggingContext);
 
-            Action<List<ProjectItemInstance>> logFunction = null;
+            Action<IList> logFunction = null;
 
             if (LogTaskInputs && !LoggingContext.LoggingService.OnlyLogCriticalEvents && itemsToAdd?.Count > 0)
             {
@@ -232,7 +233,7 @@ private void ExecuteAdd(ProjectItemGroupTaskItemInstance child, ItemBucket bucke
                         child.Location);
                 };
             }
-           
+
             // Now add the items we created to the lookup.
             bucket.Lookup.AddNewItemsOfItemType(child.ItemType, itemsToAdd, !keepDuplicates, logFunction);
             // Add in one operation for potential copy-on-write

src/Build/BackEnd/Components/RequestBuilder/Lookup.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
+using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
@@ -636,7 +637,7 @@ internal void SetProperty(ProjectPropertyInstance property)
         /// <summary>
         /// Implements a true add, an item that has been created in a batch.
         /// </summary>
-        internal void AddNewItemsOfItemType(string itemType, ICollection<ProjectItemInstance> group, bool doNotAddDuplicates = false, Action<List<ProjectItemInstance>> logFunction = null)
+        internal void AddNewItemsOfItemType(string itemType, ICollection<ProjectItemInstance> group, bool doNotAddDuplicates = false, Action<IList> logFunction = null)
         {
             // Adding to outer scope could be easily implemented, but our code does not do it at present
             MustNotBeOuterScope();
@@ -658,24 +659,29 @@ internal void AddNewItemsOfItemType(string itemType, ICollection<ProjectItemInst
             IEnumerable<ProjectItemInstance> itemsToAdd = group;
             if (doNotAddDuplicates)
             {
-                // Remove duplicates from the inputs.
-                itemsToAdd = itemsToAdd.Distinct(ProjectItemInstance.EqualityComparer);
 
                 // Ensure we don't also add any that already exist.
                 var existingItems = GetItems(itemType);
 
                 var existingItemsHashSet = existingItems.ToHashSet(ProjectItemInstance.EqualityComparer);
-                
+
                 if (existingItems.Count > 0)
                 {
-                    
+
                     var deduplicatedItemsToAdd = new List<ProjectItemInstance>();
-                    foreach (var item in itemsToAdd) {
-                        if (!existingItemsHashSet.Contains(item)) {
+                    foreach (var item in itemsToAdd)
+                    {
+                        if (existingItemsHashSet.Add(item))
+                        {
                             deduplicatedItemsToAdd.Add(item);
                         }
                     }
-                   itemsToAdd = deduplicatedItemsToAdd;
+                    itemsToAdd = deduplicatedItemsToAdd;
+                }
+                else
+                {
+                    // Remove the duplicates in case we're not concerned with the existing items.
+                    itemsToAdd = itemsToAdd.Distinct(ProjectItemInstance.EqualityComparer);
                 }
             }
 
@@ -687,7 +693,6 @@ internal void AddNewItemsOfItemType(string itemType, ICollection<ProjectItemInst
                 var groupAsList = group as List<ProjectItemInstance>;
                 logFunction?.Invoke(groupAsList ?? group.ToList());  
             }
-           
 
             PrimaryAddTable.ImportItemsOfType(itemType, itemsToAdd);
         }