import 'package:flutter/material.dart';
import 'package:teste/Screen1.dart';
import 'package:teste/Screen2.dart';
void main() {
  runApp(MyApp());
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      home: MyHomePage(title: 'Flutter Demo Home Page'),
class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;
  @override
  _MyHomePageState createState() => _MyHomePageState();
GlobalKey<ScaffoldState> _globalKeyScaffold = GlobalKey<ScaffoldState>();
class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      key: _globalKeyScaffold,
      backgroundColor: Colors.red,
      body: Center(
        child: Stack(
          children: <Widget>[
            Hero(
              tag: "placeholder",
              child: Placeholder()),
            Center(child: 
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
              Hero(
                tag: "screen1",
                              child: RaisedButton(onPressed: (){
                  Navigator.push(context, MaterialPageRoute(builder: (context)=>Screen1()));
                }, child: Text("Screen1"),),
              Hero(
                tag: "screen2",
                child: RaisedButton(onPressed: (){
                          Navigator.push(context, MaterialPageRoute(builder: (context)=>Screen2()));
                }, child: Text("Screen2"),)),
Screen1.dart
import 'package:flutter/material.dart';
class Screen1 extends StatefulWidget {
  @override
  _Screen1State createState() => _Screen1State();
GlobalKey<ScaffoldState> _globalKeyScaffold1 = GlobalKey<ScaffoldState>();
GlobalKey<FormState> _formKey1 = GlobalKey<FormState>(debugLabel: "1");
class _Screen1State extends State<Screen1> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      key: _globalKeyScaffold1,
      backgroundColor: Colors.blue,
      body: Stack(
        alignment: Alignment.bottomLeft,
        children: <Widget>[
        Hero(
          tag: "placeholder",
          child: Placeholder()),
        Hero(
          tag: "screen1",
                  child: Container(
          color: Colors.white,
          height: MediaQuery.of(context).size.height/2, 
          width: MediaQuery.of(context).size.width-100,
          child: Form(
            key: _formKey1,
            child: Column(children: <Widget>[
            TextFormField(
              validator: (string){
               if(string.length<5){
                 return "Ok";
            TextFormField(),
            TextFormField(),
            RaisedButton(onPressed: (){
              _formKey1.currentState.validate();
          ],)),),
Screen2.dart
import 'package:flutter/material.dart';
class Screen2 extends StatefulWidget {
  @override
  _Screen2State createState() => _Screen2State();
GlobalKey<ScaffoldState> _globalKeyScaffold2 = GlobalKey<ScaffoldState>();
GlobalKey<FormState> _formKey2 = GlobalKey<FormState>(debugLabel: "2");
class _Screen2State extends State<Screen2> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      key: _globalKeyScaffold2,
      backgroundColor: Colors.green,
      body: Stack(
        alignment: Alignment.bottomRight,
        children: <Widget>[
        Hero(
          tag: "placeholder",
          child: Placeholder()),
        Hero(
          tag: "screen2",
                  child: Container(
          color: Colors.white,
          height: MediaQuery.of(context).size.height/2, 
          width: MediaQuery.of(context).size.width-100,
          child: Form(
            key: _formKey2,
            child: Column(children: <Widget>[
            TextFormField(
               validator: (string){
               if(string.length<5){
                 return "Ok";
            TextFormField(),
            TextFormField(),
             RaisedButton(onPressed: (){
              _formKey2.currentState.validate();
          ],)),),
flutter doctor (when isn't working)
[√] Flutter (Channel stable, v1.17.5, on Microsoft Windows [versão 10.0.18362.900], locale pt-BR)
[√] Android toolchain - develop for Android devices (Android SDK version 28.0.3)
[√] Android Studio (version 3.5)
[√] VS Code, 64-bit edition (version 1.46.1)
[√] Connected device (1 available)
• No issues found!
flutter doctor (working)
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, v1.17.3, on Mac OS X 10.14.6 18G103, locale pt-BR)
[✓] Android toolchain - develop for Android devices (Android SDK version 29.0.3)
[✓] Xcode - develop for iOS and macOS (Xcode 11.3.1)
[✓] Android Studio (version 3.5)
[✓] VS Code (version 1.46.1)
[!] Connected device
    ! No devices available
! Doctor found issues in 1 category.
(On Windows also work when I downgrade do 1.14.6 and on Mac also doesn't work when I update to 1.17)
  waiting for customer response
  The Flutter team cannot make further progress on this issue until the original reporter responds
 label
    Jul 9, 2020
          
Hi @equisbacon,

I looked at your code and observed that the below code snippet is causing the issue you are seeing:
child: Form(
                  key: _formKey1,
                  child: Column(children: <Widget>[
                    TextFormField(
                      validator: (string){
                        if(string.length<5){
                          return "Ok";
                        return null;
                    TextFormField(),
                    TextFormField(),
                    RaisedButton(onPressed: (){
                      _formKey1.currentState.validate();
                  ],)),
For some reason, when you go back from screen 1, the screen  (screen 1) isn't disposed and hence the _globalKeyScaffold1 still is in cache and when are on main screen (ie the screen which shows screen1 and screen2), the _globakeyScaffold is detected along with _globalKeyScaffold1 and hence the error 'duplicate globalkeys detected.

I think this seem to be a coding issue wherein you'll need to dispose the state of the screens when they navigate to and fro.
You may also get some help if you post it on Stack Overflow and if you need help with your code, please see https://www.reddit.com/r/flutterhelp/

Closing, as this isn't an issue with Flutter itself. If you disagree, please write in the comments and I will reopen it.

Thank you