fixes navigator to be able to handle route with duplicate page key in… (#97394)

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

@@ -2801,7 +2801,7 @@ class _RouteEntry extends RouteTransitionRecord {
   bool get hasPage => route.settings is Page;
 
   bool canUpdateFrom(Page<dynamic> page) {
-    if (currentState.index > _RouteLifecycle.idle.index)
+    if (!willBePresent)
       return false;
     if (!hasPage)
       return false;
@@ -3635,6 +3635,9 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin, Res
     // Scans middle of the old entries and records the page key to old entry map.
     int oldEntriesBottomToScan = oldEntriesBottom;
     final Map<LocalKey, _RouteEntry> pageKeyToOldEntry = <LocalKey, _RouteEntry>{};
+    // This set contains entries that are transitioning out but are still in
+    // the route stack.
+    final Set<_RouteEntry> phantomEntries = <_RouteEntry>{};
     while (oldEntriesBottomToScan <= oldEntriesTop) {
       final _RouteEntry oldEntry = _history[oldEntriesBottomToScan];
       oldEntriesBottomToScan += 1;
@@ -3650,9 +3653,14 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin, Res
       assert(oldEntry.hasPage);
 
       final Page<dynamic> page = oldEntry.route.settings as Page<dynamic>;
+
       if (page.key == null)
         continue;
 
+      if (!oldEntry.willBePresent) {
+        phantomEntries.add(oldEntry);
+        continue;
+      }
       assert(!pageKeyToOldEntry.containsKey(page.key));
       pageKeyToOldEntry[page.key!] = oldEntry;
     }
@@ -3703,21 +3711,21 @@ class NavigatorState extends State<Navigator> with TickerProviderStateMixin, Res
             () => <_RouteEntry>[],
           );
         pagelessRoutes.add(potentialEntryToRemove);
-        if (previousOldPageRouteEntry!.isWaitingForExitingDecision && potentialEntryToRemove.isPresent)
+        if (previousOldPageRouteEntry!.isWaitingForExitingDecision && potentialEntryToRemove.willBePresent)
           potentialEntryToRemove.markNeedsExitingDecision();
         continue;
       }
 
       final Page<dynamic> potentialPageToRemove = potentialEntryToRemove.route.settings as Page<dynamic>;
       // Marks for transition delegate to remove if this old page does not have
-      // a key or was not taken during updating the middle of new page.
-      if (
-        potentialPageToRemove.key == null ||
-        pageKeyToOldEntry.containsKey(potentialPageToRemove.key)
-      ) {
+      // a key, was not taken during updating the middle of new page, or is
+      // already transitioning out.
+      if (potentialPageToRemove.key == null ||
+          pageKeyToOldEntry.containsKey(potentialPageToRemove.key) ||
+          phantomEntries.contains(potentialEntryToRemove)) {
         locationToExitingPageRoute[previousOldPageRouteEntry] = potentialEntryToRemove;
         // We only need a decision if it has not already been popped.
-        if (potentialEntryToRemove.isPresent)
+        if (potentialEntryToRemove.willBePresent)
           potentialEntryToRemove.markNeedsExitingDecision();
       }
       previousOldPageRouteEntry = potentialEntryToRemove;

packages/flutter/test/widgets/navigator_test.dart

@@ -2644,6 +2644,42 @@ void main() {
       expect(find.text('initial'), findsOneWidget);
     });
 
+    testWidgets('can handle duplicate page key if update before transition finishes', (WidgetTester tester) async {
+      // Regression test for https://github.com/flutter/flutter/issues/97363.
+      final GlobalKey<NavigatorState> navigator = GlobalKey<NavigatorState>();
+      final List<TestPage> myPages1 = <TestPage>[
+        const TestPage(key: ValueKey<String>('1'), name:'initial'),
+      ];
+      final List<TestPage> myPages2 = <TestPage>[
+        const TestPage(key: ValueKey<String>('2'), name:'second'),
+      ];
+
+      bool onPopPage(Route<dynamic> route, dynamic result) => false;
+
+      await tester.pumpWidget(
+        buildNavigator(pages: myPages1, onPopPage: onPopPage, key: navigator),
+      );
+      await tester.pump();
+      expect(find.text('initial'), findsOneWidget);
+      // Update multiple times without waiting for pop to finish to leave
+      // multiple popping route entries in route stack with the same page key.
+      await tester.pumpWidget(
+        buildNavigator(pages: myPages2, onPopPage: onPopPage, key: navigator),
+      );
+      await tester.pump();
+      await tester.pumpWidget(
+        buildNavigator(pages: myPages1, onPopPage: onPopPage, key: navigator),
+      );
+      await tester.pump();
+      await tester.pumpWidget(
+        buildNavigator(pages: myPages2, onPopPage: onPopPage, key: navigator),
+      );
+
+      await tester.pumpAndSettle();
+      expect(find.text('second'), findsOneWidget);
+      expect(tester.takeException(), isNull);
+    });
+
     testWidgets('throw if onPopPage callback is not provided', (WidgetTester tester) async {
       final List<TestPage> myPages = <TestPage>[
         const TestPage(key: ValueKey<String>('1'), name:'initial'),