Fix flaky File_Move_Multiple_From_Watched_To_Unwatched_Mac test caused by duplicate FSEvents (#125779)

macOS FSEvents can deliver duplicate `Deleted` events when moving
files/directories from a watched to an unwatched directory. The existing
`isFilteredOut` filter (Mac path, `skipOldEvents=true`) only discarded
stale `Created`/`Changed` events—not duplicate `Deleted` events—causing
`ExpectEvents` to signal completion prematurely and collect an extra
event:

```
Expected: [Deleted file0, Deleted file1]
Actual:   [Deleted file0, Deleted file0, Deleted file1]
```

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jtschuster <36744439+jtschuster@users.noreply.github.com>

Author: Copilot

src/libraries/System.IO.FileSystem.Watcher/tests/FileSystemWatcher.Directory.Move.cs

@@ -173,7 +173,10 @@ private void DirectoryMove_Multiple_FromWatchedToUnwatched(int filesCount, bool
             Action action = () => Array.ForEach(dirs, dir => Directory.Move(dir.DirectoryInWatchedDir, dir.DirectoryInUnwatchedDir));
 
             // Filter out Created events as there is a race-condition when moving a directory and then observing a parent folder. It receives Create event although Watcher is not registered yet.
-            Func<FiredEvent, bool>? isFilteredOut = skipOldEvents ? x => x.EventType == WatcherChangeTypes.Created : null;
+            // Also filter out duplicate events as Mac FSEvents can deliver the same Deleted event multiple times.
+            Func<FiredEvent, bool>? isFilteredOut = skipOldEvents
+                ? CreateDeduplicatingFilter(WatcherChangeTypes.Created)
+                : null;
 
             IEnumerable<FiredEvent> events = ExpectEvents(watcher, filesCount, action, isFilteredOut);
 

src/libraries/System.IO.FileSystem.Watcher/tests/FileSystemWatcher.File.Move.cs

@@ -225,7 +225,10 @@ private void FileMove_Multiple_FromWatchedToUnwatched(int filesCount, bool skipO
             Action action = () => Array.ForEach(files, file => File.Move(file.FileInWatchedDir, file.FileInUnwatchedDir));
 
             // Filter out Created and Changed events as there is a race-condition when moving a file and then observing a parent folder. It receives Create and Changed events although Watcher is not registered yet.
-            Func<FiredEvent, bool>? isFilteredOut = skipOldEvents ? x => (x.EventType & (WatcherChangeTypes.Created | WatcherChangeTypes.Changed)) != 0 : null;
+            // Also filter out duplicate events as Mac FSEvents can deliver the same Deleted event multiple times.
+            Func<FiredEvent, bool>? isFilteredOut = skipOldEvents
+                ? CreateDeduplicatingFilter(WatcherChangeTypes.Created | WatcherChangeTypes.Changed)
+                : null;
 
             IEnumerable<FiredEvent> events = ExpectEvents(watcher, filesCount, action, isFilteredOut);
 

src/libraries/System.IO.FileSystem.Watcher/tests/Utility/FileSystemWatcherTest.cs

@@ -1,6 +1,7 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Diagnostics;
@@ -505,6 +506,16 @@ public bool Equals(FiredEvent other) => EventType == other.EventType &&
 
         }
 
+        // Returns a predicate that returns true for events that should be filtered out.
+        // Events whose type matches filteredTypes are always filtered; remaining events are deduplicated
+        // so that only the first occurrence passes through.
+        // Used on platforms like macOS where FSEvents may deliver the same event more than once.
+        internal static Func<FiredEvent, bool> CreateDeduplicatingFilter(WatcherChangeTypes filteredTypes = 0)
+        {
+            var seenEvents = new ConcurrentDictionary<FiredEvent, bool>();
+            return firedEvent => (firedEvent.EventType & filteredTypes) != 0 || !seenEvents.TryAdd(firedEvent, true);
+        }
+
         // Observe until an expected count of events is triggered, otherwise fail. Return all filtered events.
         internal static List<FiredEvent> ExpectEvents(FileSystemWatcher watcher, int expectedEvents, Action action, Func<FiredEvent, bool>? isFilteredOut = null)
         {