Firebase, login by same email different provider

This is supported by Firebase Auth through the "Prevent creation of multiple accounts with the same email address" setting. However, Google is a special case as it is a verified provider and will overwrite unverified accounts with the same email. For example, if an email/password account is created and not verified and then a user signs in with Google with the same email, the old password is overridden and unlinked but the same user (same uid) is returned.

For other cases, this is how it is handled.

Let's say you sign in with Email/Password using an email/password with account [email protected]. A user then tries to sign in with a Facebook provider using the same email. The Auth backend will throw an error and linking will be required. After that is done, a user can sign in with either accounts.

Here is an example:

    var existingEmail = null;
    var pendingCred = null;
    var facebookProvider = new firebase.auth.FacebookAuthProvider();
    firebase.auth().signInWithPopup(facebookProvider)
      .then(function(result) {
        // Successful sign-in.
      });
      .catch(function(error) {
        // Account exists with different credential. To recover both accounts
        // have to be linked but the user must prove ownership of the original
        // account.
        if (error.code == 'auth/account-exists-with-different-credential') {
          existingEmail = error.email;
          pendingCred = error.credential;
          // Lookup existing account’s provider ID.
          return firebase.auth().fetchProvidersForEmail(error.email)
            .then(function(providers) {
               if (providers.indexOf(firebase.auth.EmailAuthProvider.PROVIDER_ID) != -1) {
                 // Password account already exists with the same email.
                 // Ask user to provide password associated with that account.
                 var password = window.prompt('Please provide the password for ' + existingEmail);
                 return firebase.auth().signInWithEmailAndPassword(existingEmail, password);    
               } else if (providers.indexOf(firebase.auth.GoogleAuthProvider.PROVIDER_ID) != -1) {
                 var googProvider = new firebase.auth.GoogleAuthProvider();
                 // Sign in user to Google with same account.
                 provider.setCustomParameters({'login_hint': existingEmail});
                 return firebase.auth().signInWithPopup(googProvider).then(function(result) {
                   return result.user;
                 });
               } else {
                 ...
               }
            })
            .then(function(user) {
              // Existing email/password or Google user signed in.
              // Link Facebook OAuth credential to existing account.
              return user.linkWithCredential(pendingCred);
            });
        }
        throw error;
      });