Firebase App named '[DEFAULT]' already exists with different options or config

I'm building an app with NextJS, NextAuth and Firebase.

While implementing NextAuth, I've encountered this error:

> error - FirebaseError: Firebase: Firebase App named '[DEFAULT]' already exists with different options or config (app/duplicate-app).

Here's my code:

<br/>

[...NextAuth].js

import NextAuth from "next-auth/next";
import GoogleProvider from "next-auth/providers/google";
import { FirestoreAdapter } from "@next-auth/firebase-adapter";

import { db } from "@/firebase/config";

export default NextAuth({
  providers: [
    GoogleProvider({
      clientId: process.env.GOOGLE_ID,
      clientSecret: process.env.GOOGLE_SECRET,
    }),
  ],
  adapter: FirestoreAdapter(db),
});

My firebase config file

import { initializeApp, getApp, getApps } from "firebase/app";
import "firebase/auth";
import { getFirestore } from "firebase/firestore";


const firebaseConfig = {
  apiKey: ___,
  authDomain: ___,
  projectId: ___,
  storageBucket: ___,
  messagingSenderId: ___,
  appId: ___,
  measurementId: ___,
};

const app =
  getApps().length === 0
    ? initializeApp({ ...firebaseConfig, projectId: firebaseConfig?.projectId })
    : getApp();
const db = getFirestore(app);

export { app, db };

As you can see in my config file, I'm testing if an app already exists, but it doesn't seem to work.

I've checked if somebody already had the same problem as me, but I didn't find an answer.

Any idea?

Thanks a lot,
Gabriel

According to the documentation for the Firebase Adapter for NextAuth.js, you should be passing in the configuration object to the adapter directly or an instance of Firestore from the Admin SDK (i.e. using import { getFirestore } from "firebase-admin/firestore").

Initially, you should try removing your "@/firebase/config" import and just use the configuration directly.

import NextAuth from "next-auth/next";
import GoogleProvider from "next-auth/providers/google";
import { FirestoreAdapter } from "@next-auth/firebase-adapter";

const firebaseConfig = {
  apiKey: ___,
  authDomain: ___,
  projectId: ___,
  storageBucket: ___,
  messagingSenderId: ___,
  appId: ___,
  measurementId: ___,
};

export default NextAuth({
  providers: [
    GoogleProvider({
      clientId: process.env.GOOGLE_ID,
      clientSecret: process.env.GOOGLE_SECRET,
    }),
  ],
  adapter: FirestoreAdapter(firebaseConfig),
});

This initialization behaviour is defined in the main constructor in src/index.ts and in the initialization utility method in src/utils.ts.

The next step to try would be to make sure your Next.js sources make use of the Firebase Admin SDK instead of the client-side Firebase SDK which behaves differently.

If the above doesn't work, you can look for the more general cause. Sift through your codebase and look for statements that initialize Firebase service providers before your code pulls in "@/firebase/config". Any call to getApp() without any arguments will silently initialize the default FirebaseApp instance.

// NOTE: this is pseudo-code, not the actual implementation
// gets the named/default app, throwing an error if not initialized
export function getApp(name: string = DEFAULT_ENTRY_NAME): FirebaseApp {
  const app = _apps.get(name);
  if (!app && name === DEFAULT_ENTRY_NAME) return initializeApp(); // <-- this initializeApp is your problem
  if (!app) throw new Error(name + " not initialized");
  return app;
}

This also applies to calls that initialize a service (e.g. getFirestore()) without any app argument as they also will call getApp() internally.

// NOTE: this is pseudo-code, not the actual implementation
export function getFirestore(app?: FirebaseApp) {
    app = app || getApp(); // use given app or use default
    return app._providers.get('firestore') || initializeFirestore(app, DEFAULT_SETTINGS)
}

Unfortunately, tracking down this particular problem can be a pain as you module bundler/build tool might be "tree-shaking" the code and stripping what it thinks is unnecessary - which may include your getApp() and getFirestore() calls if you don't use app or db in the local code. Using just import "@/firebase/config" in this case should solve that.