In simple registrations this is fine. In multi-page registrations with long forms, it can waste the user's time by making them input all the data again when it wasn't needed, though.
Except in really critical applications like payments, etc., as a user I'd rather have the convenience of being warned early about already having an account that the little extra security.
Except in really critical applications like payments, etc., as a user I'd rather have the convenience of being warned early about already having an account that the little extra security.