standalone fully functioning usage of https://gist.github.com/micimize/36762485690b8423b553bd095654dfa7

cheaters_global_key_full_example.dart

/*

  FULL cheaters_global_key.dart example, 
  contents:
  * first defines the cheaters_global_key,
  * then a few helpers
  * then at the bottom a usage example

    BEGIN cheaters_global_keys.dart 
*/

import 'package:flutter/material.dart';

class CheatersGlobalKey extends InheritedWidget {
  CheatersGlobalKey._({
    this.cheatersKeys = const {},
    Key key,
    Widget child,
  }) : super(key: key, child: child);

  final Map<String, Key> cheatersKeys;

  @override
  bool updateShouldNotify(CheatersGlobalKey oldWidget) {
    return cheatersKeys != oldWidget.cheatersKeys;
  }

  static Key of(
    BuildContext context,
    String label,
  ) {
    assert(context != null);

    final route = ModalRoute.of(context);

    // only the current route cans safely render a GlobalKey
    if (!route.isCurrent) {
      return Key(label);
    }

    // can't render a GlobalKey if it ain't there -
    // this ensures the current tab gets the global key
    final CheatersGlobalKey result =
        context.dependOnInheritedWidgetOfExactType<CheatersGlobalKey>();

    if (result == null) {
      return Key(label);
    }

    final keys = result?.cheatersKeys ?? {};
    final key = keys[label] ??
        Key(label); // we default to safely returning a regular key

    return key;
  }
}

class CheatersIndexedStack extends StatelessWidget {
  CheatersIndexedStack({
    Key key,
    @required this.index,
    @required this.cheatersKeys,
    @required this.children,
    List<String> contextLabels,
  })  : this.contextLabels = contextLabels ??
            List<String>.generate(
              children.length,
              (i) => 'cheater.index=$i',
            ),
        super(key: key) {
    assert(this.contextLabels.length == children.length);
  }

  final Set<String> cheatersKeys;
  final List<String> contextLabels;
  final List<Widget> children;
  final int index;

  Map<String, Key> keysFor(int childIndex) {
    return index == childIndex
        ? Map.fromEntries(cheatersKeys.map(
            (label) => MapEntry(
              label,
              _CheatersGlobalKey(label, contextLabels[childIndex]),
            ),
          ))
        : null;
  }

  @override
  Widget build(BuildContext context) {
    return IndexedStack(
      index: index,
      children: children.map(withIndex((child, childIndex) {
        return CheatersGlobalKey._(
          // should change every tab switch
          cheatersKeys: keysFor(childIndex),
          child: child,
        );
      })).toList(),
    );
  }
}

class _CheatersGlobalKey extends GlobalObjectKey {
  const _CheatersGlobalKey(String globalLabel, this.contextLabel)
      : super(globalLabel);

  final String contextLabel;

  @override
  String toString() {
    return '[_CheatersGlobalKey#$hashCode value=$value, context=$contextLabel]';
  }
}

/* 
    END cheaters_global_keys.dart 


    BEGIN pointless_helpers.dart
*/

typedef MapFn<A, B> = B Function(A a);

typedef IndexedMapFn<A, B> = B Function(A a, int index);

MapFn<A, B> withIndex<A, B>(IndexedMapFn<A, B> mapFn) {
  int index = -1;
  return (A a) => mapFn(a, ++index);
}

/* 
    END pointless_helpers.dart


    BEGIN full_example.dart 
*/

class TabBarExample extends StatefulWidget {
  const TabBarExample({
    Key key,
  }) : super(key: key);

  @override
  _TabBarExampleState createState() => _TabBarExampleState();
}

class _TabBarExampleState extends State<TabBarExample>
    with TickerProviderStateMixin {
  TabController tabController;
  int tabIndex = 0;

  @override
  void initState() {
    super.initState();

    tabController = TabController(
      initialIndex: 0,
      length: 3,
      vsync: this,
    );
  }

  static final tabs = [
    'ONE',
    "TWO",
    "THREE",
  ]
      .map((t) => Padding(
            padding: const EdgeInsets.all(16.0),
            child: Text(t),
          ))
      .toList();

  Widget tabOf(BuildContext context, int tabIndex) {
    final key = CheatersGlobalKey.of(context, 'tabs');
    return Column(mainAxisSize: MainAxisSize.max, children: [
      TabBar(
        key: CheatersGlobalKey.of(context, 'tabs'),
        onTap: onTapNav,
        controller: tabController,
        tabs: tabs,
      ),
      Container(
        height: 300,
        width: 300,
        child: Center(
          child: Text(
            key.toString(),
            style: Theme.of(context)
                .textTheme
                .button
                .copyWith(color: Colors.white),
          ),
        ),
      ),
    ]);
  }

  Widget pageContent(BuildContext context, int selectedIndex) =>
      CheatersIndexedStack(
        index: selectedIndex,
        cheatersKeys: const {'tabs'},
        children: List.generate(
          tabs.length,
          (tabIndex) => Builder(
            builder: (context) => tabOf(context, tabIndex),
          ),
        ),
      );

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.grey.shade800,
      appBar: AppBar(
        backgroundColor: Colors.indigo,
        title: Text('AppBar!'),
      ),
      body: pageContent(context, tabIndex),
    );
  }

  void onTapNav(int index) {
    if (mounted) {
      setState(() {
        tabIndex = index;
      });
    }
  }
}

void main() {
  runApp(
    MaterialApp(
      debugShowCheckedModeBanner: false,
      home: TabBarExample(),
    ),
  );
}

I was getting a pretty inscrutable error from this recently, and I think it was because there was a structure like ImplicitlyAnimated#CheatersGlobalKeyA(CheatersGlobalKeyB)). I believe the reason this started creating a problem was because flutter made widget disposal more optimal or something, but basically:

• the top level widget is getting transitioned out, so it's children aren't updated
• this means the nested global key isn't replaced, so a duplicate key error is thrown.

But, the tab animations seem fine, meaning there isn't actually a need to nest these global keys (for now)