ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: [core/duplicate-app] A Firebase App named "[DEFAULT]" already exists

Add this in Flutter

NOTE - Use this "Firebase.initializeApp" only Once in the entire Project.

// Wait for Default Firebase app to initialize

if (Firebase.apps.isEmpty) {
  await Firebase.initializeApp(
  name: 'YourAPP',
  options: DefaultFirebaseOptions.currentPlatform,
 ).whenComplete(() {
  print("completedAppInitialize");
 });
}

I solved the issue by adding a name to the initializeApp method:

await Firebase.initializeApp(
  name: 'name-here',
  options: DefaultFirebaseOptions.currentPlatform,
)

I used the same name used in the firebase console as my project ID for consistency, keep in mind that you cannot add spaces to the name (only dashes -).

flutter clean

and

flutter pub get

executing these two commands consecutively solved my problem.