Remerge "Fixed AnimatedSwitcher chain produced duplicates" after fixing issues with g3

This reverts commit 1d2fa285.

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

@@ -222,6 +222,7 @@ 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,
     );
@@ -394,6 +395,6 @@ class _AnimatedSwitcherState extends State<AnimatedSwitcher> with TickerProvider
   @override
   Widget build(BuildContext context) {
     _rebuildOutgoingWidgetsIfNeeded();
-    return widget.layoutBuilder(_currentEntry?.transition, _outgoingWidgets!);
+    return widget.layoutBuilder(_currentEntry?.transition, _outgoingWidgets!.where((Widget outgoing) => outgoing.key != _currentEntry?.transition.key).toSet().toList());
   }
 }

packages/flutter/test/widgets/animated_switcher_test.dart

@@ -415,6 +415,25 @@ void main() {
       );
     }
   });
+
+  testWidgets('AnimatedSwitcher does not duplicate animations if the same child is entered twice.', (WidgetTester tester) async {
+    Future<void> pumpChild(Widget child) async {
+      return tester.pumpWidget(
+        Directionality(
+          textDirection: TextDirection.ltr,
+          child: AnimatedSwitcher(
+            duration: const Duration(milliseconds: 1000),
+            child: child,
+          ),
+        ),
+      );
+    }
+    await pumpChild(const Text('1', key: Key('1')));
+    await pumpChild(const Text('2', key: Key('2')));
+    await pumpChild(const Text('1', key: Key('1')));
+    await tester.pump(const Duration(milliseconds: 1000));
+    expect(find.text('1'), findsOneWidget);
+  });
 }
 
 class StatefulTest extends StatefulWidget {