Use AnimatedSwitcher's _childNumber as Key in layoutBuilder's Stack

packages/flutter/lib/src/widgets/animated_switcher.dart

@@ -216,7 +216,6 @@ class AnimatedSwitcher extends StatefulWidget {
   /// This is an [AnimatedSwitcherTransitionBuilder] function.
   static Widget defaultTransitionBuilder(Widget child, Animation<double> animation) {
     return FadeTransition(
-      key: ValueKey<Key?>(child.key),
       opacity: animation,
       child: child,
     );
@@ -338,7 +337,10 @@ class _AnimatedSwitcherState extends State<AnimatedSwitcher> with TickerProvider
   }) {
     final _ChildEntry entry = _ChildEntry(
       widgetChild: child,
-      transition: KeyedSubtree.wrap(builder(child, animation), _childNumber),
+      transition: KeyedSubtree(
+        key: ValueKey<int>(_childNumber),
+        child: builder(child, animation),
+      ),
       animation: animation,
       controller: controller,
     );
@@ -389,6 +391,6 @@ class _AnimatedSwitcherState extends State<AnimatedSwitcher> with TickerProvider
   @override
   Widget build(BuildContext context) {
     _rebuildOutgoingWidgetsIfNeeded();
-    return widget.layoutBuilder(_currentEntry?.transition, _outgoingWidgets!.where((Widget outgoing) => outgoing.key != _currentEntry?.transition.key).toSet().toList());
+    return widget.layoutBuilder(_currentEntry?.transition, _outgoingWidgets!);
   }
 }

packages/flutter/test/widgets/animated_switcher_test.dart

@@ -416,23 +416,44 @@ void main() {
     }
   });
 
-  testWidgets('AnimatedSwitcher does not duplicate animations if the same child is entered twice.', (WidgetTester tester) async {
-    Future<void> pumpChild(Widget child) async {
+  testWidgets('AnimatedSwitcher can handle multiple children with the same key.', (WidgetTester tester) async {
+    final UniqueKey containerA = UniqueKey();
+    final UniqueKey containerB = UniqueKey();
+
+    // Pump an AnimatedSwitcher with a child container with the given key.
+    Future<void> pump(Key key) async {
       return tester.pumpWidget(
-        Directionality(
-          textDirection: TextDirection.ltr,
-          child: AnimatedSwitcher(
-            duration: const Duration(milliseconds: 1000),
-            child: child,
-          ),
+        AnimatedSwitcher(
+          duration: const Duration(milliseconds: 1000),
+          child: Container(key: key),
         ),
       );
     }
-    await pumpChild(const Text('1', key: Key('1')));
-    await pumpChild(const Text('2', key: Key('2')));
-    await pumpChild(const Text('1', key: Key('1')));
+
+    // Pump four widgets with the two keys A and B in alternating order.
+    await pump(containerA);
     await tester.pump(const Duration(milliseconds: 1000));
-    expect(find.text('1'), findsOneWidget);
+    await pump(containerB);
+    await tester.pump(const Duration(milliseconds: 500));
+    await pump(containerA);
+    await tester.pump(const Duration(milliseconds: 250));
+    await pump(containerB);
+    await tester.pump(const Duration(milliseconds: 125));
+
+    // All four widgets should still be animating in (the one pumped last) or
+    // out (the other ones), and thus have an associated FadeTransition each.
+    expect(find.byType(FadeTransition), findsNWidgets(4));
+    final Iterable<FadeTransition> transitions = tester.widgetList(
+      find.byType(FadeTransition),
+    );
+
+    // The exponentially decaying timing used in pumping the widgets should have
+    // lined up all of the FadeTransitions' values to be the same.
+    for (final FadeTransition transition in transitions) {
+      expect(transition.opacity.value, moreOrLessEquals(0.125, epsilon: 0.001));
+    }
+
+    await tester.pumpAndSettle();
   });
 }