Minimal code example resulting in an error saying multiple widgets used the same GlobalKey.

multiple_widgets_used_same_globalkey.dart

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: LandingPage(),
    );
  }
}

// Three final GlobalKeys
final List<GlobalKey<FormState>> _formKeys = [
  GlobalKey<FormState>(),
  GlobalKey<FormState>(),
  GlobalKey<FormState>()
];

// The page where users can create a new account by filling in email, name, etc.
class RegisterPage extends StatelessWidget {
  RegisterPage({@required this.registerFormKey});

  final GlobalKey<FormState> registerFormKey;

  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.grey,
      child: Form(
        key: registerFormKey, // Line presumably throwing the error
        child: AutofillGroup(
          child: Column(
            children: [
              Text("Register"),
              TextFormField(),
            ],
          ),
        ),
      ),
    );
  }
}

// The page where users can login with their email and password
class LoginPage extends StatelessWidget {
  LoginPage({@required this.loginFormKey});

  final GlobalKey<FormState> loginFormKey;

  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.blue,
      child: Form(
        key: loginFormKey, // Line presumably throwing the error
        child: AutofillGroup(
          child: Column(
            children: [
              Text("Log in"),
              TextFormField(),
            ],
          ),
        ),
      ),
    );
  }
}

// The page where users can reset their password
class ResetPage extends StatelessWidget {
  ResetPage({@required this.passwordFormKey});

  final GlobalKey<FormState> passwordFormKey;

  @override
  Widget build(BuildContext context) {
    return Container(
      color: Colors.orange,
      child: Form(
        key: passwordFormKey, // Line presumably throwing the error
        child: AutofillGroup(
          child: Column(
            children: [
              Text("Reset password"),
              TextFormField(),
            ],
          ),
        ),
      ),
    );
  }
}

// The app's landing page, containing an animated IndexedStack that navigates
// through all three of the above declared pages.
class LandingPage extends StatefulWidget {
  // The pages are declared as final variables
  final RegisterPage registerPage = RegisterPage(registerFormKey: _formKeys[0]);
  final LoginPage loginPage = LoginPage(loginFormKey: _formKeys[1]);
  final ResetPage resetScreen = ResetPage(passwordFormKey: _formKeys[2]);

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

class _LandingPageState extends State<LandingPage> {
  int view = 1;
  int previousView = 1; // Used for animation (the error breaks it)

  @override
  Widget build(BuildContext context) {
    print(view);
    return Scaffold(
      body: Column(
        mainAxisAlignment: MainAxisAlignment.center,
        mainAxisSize: MainAxisSize.max,
        children: [
          AnimatedSwitcher(
            transitionBuilder: (child, animation) =>
              transitionBuilder(animation, child, view, previousView),
            duration: Duration(milliseconds: 200),
            child: IndexedStack(
              key: ValueKey<int>(view),
              index: view,
              children: [
                this.widget.registerPage,
                this.widget.loginPage,
                this.widget.resetScreen,
              ],
            ),
          ),
          ButtonBar(
            children: [
              ElevatedButton(
                child: Text("Back"),
                onPressed: () {
                  if (view == 0) return null;
                  setState(() {
                    previousView = view;
                    view -= 1;
                  });
                },
              ),
              ElevatedButton(
                child: Text("Continue"),
                onPressed: () {
                  if (view == 2) return null;
                  setState(() {
                    previousView = view;
                    view += 1;
                  });
                },
              )
            ],
          )
        ],
      ),
    );
  }
}

SlideTransition transitionBuilder(
    Animation<double> animation, Widget child, int view, int previousView) {
  final fromLeft =
      Tween<Offset>(begin: Offset(-1.0, 0.0), end: Offset(0.0, 0.0))
          .animate(animation);
  final fromRight =
      Tween<Offset>(begin: Offset(1.0, 0.0), end: Offset(0.0, 0.0))
          .animate(animation);

  bool goingIn = (child.key == ValueKey(view));
  bool goingBack = (view < previousView);

  return SlideTransition(
      child: Padding(
        padding: const EdgeInsets.symmetric(horizontal: 16.0),
        child: child,
      ),
      position: (goingIn && !goingBack || !goingIn && goingBack)
          ? fromRight
          : fromLeft);
}