Cannot add "email" provider to an existing user

[ttskch]

Case with no problems

1. User signs up with email provider.
2. A record is inserted to the auth.users table and ["email"] is set in the app_metadata.providers.
3. A record is inserted to the auth.identities table.
4. User signs in with signInWithOAuth() using a Twitter account that has the same email as the one registered in step 1.
   5 A record is inserted to the auth.identities table.
5. Since the emails match, the user is considered the same and the app_metadata.providers in auth.users is updated to ["email", "twitter"].
6. From now on, the user can sign in with either email or twitter.

Case with problems

1. User signs up with a twitter provider.
2. A record is inserted to the auth.users table and ["twitter"] is set in the app_metadata.providers.
3. A record is inserted to the auth.identities table.
4. User tries to signUp() with the same email as the Twitter account used in step 1.
5. "User already registered." error occurs.
6. There is no way to add a record to the auth.identities table corresponding to the email provider for this user.

[ioucyf]

It also throws an error when a user changes their email to an unrecorded address, then tries to change it back to one that is already associated to one of their identities; which makes it impossible without admin/postgres functions.