Authentication using Facebook at first and then Google causes an error in Firebase for Android
Go to Authentication > Sign-in providers, click Multiple accounts per email address and Allow creation of multiple accounts with the same email address is what you are looking for.
Please check the thread: https://groups.google.com/forum/#!searchin/firebase-talk/liu/firebase-talk/ms_NVQem_Cw/8g7BFk1IAAAJ It explains why this happens. This is due to some security issue with Google emails being verified whereas Facebook emails are not.
I finally ended with this logic:
If user try to sign in with Facebook, but user with given email already exist (with Google provider) and this errors occures:
"An account already exists with the same email address but different sign-in credentials. Sign in using a provider associated with this email address."
So, just ask user to loging using Google (and after it silently link Facebook to existing account)