Authentication using Facebook at first and then Google causes an error in Firebase for Android

Question

As I understand from the Firebase Docs, if a user authenticates his account with a credential, he should strictly login by using the same credential if the credential is not

Answer 1

Allow creation of multiple accounts with the same email address is what you are looking for.

That works fine ONLY if you check the email in your backend and that's the reference for your users. If you use the Firebase Id then that won't enable to keep unique users.