Registration with same username causes problems on payment process

One-sentence description

Registration with same username causes problems on payment process

Current behaviour

If two registrations exist with the same entered/generated username and the first is fully processed while the second is in the accepted stage the payment for that second registration will not be able to fully succeed since the username is taken. However, the payment is marked processed. Thus the registration will be in limbo, forever.

Expected behaviour

No limbo!

Steps to reproduce

1. Create two registrations with the same first and last name
2. Accept them both
3. Process payment for 1
4. Try to process payment for 2
5. Erreur! 💥