It might be that you have a corrupt index. What version of discourse and Postgres do you have?
Do you have social logins enabled? One possibility is that they tried to sign up with the email address, didn’t respond to the validation email, and then used social login.
I’d probably try to fix it at the rails console or perhaps directly in sql.
Other than it being confusing that the address shows twice, there is no actual problem, right? The user can log in, but has an extra address in their profile.