You can configure whether users can create multiple accounts that use the same email address, but are linked to different sign-in methods. For example, if you don't allow multiple accounts with the same email address, a user cannot create a new account that signs in using a Google Account with the email address ex@gmail.com if there already is an account that signs in using the email address ex@gmail.com and a password.
If you do allow multiple accounts with the same email address, your app's sign-in flow cannot rely on an email address to identify a user account.
Allowing multiple accounts to have the same email address also changes the behavior of the following functions in the Web SDK:
firebase.auth().signInWithPopup()firebase.auth().signInWithRedirect()firebase.auth().linkWithPopup()firebase.auth().linkWithRedirect()
When you allow only one account per email address, these functions retrieve user profile information from the identity providers. This profile information includes the user's email address, display name, and the URL to the user's profile photo. If you change the default setting by allowing multiple accounts to have the same email address, these functions will not retrieve profile information; if you need this information, you must retrieve it from the identity provider yourself.
Users can never create multiple accounts with the same email address and sign-in method.