re-use username or e-mail that has been deleted

Hi

I hope you have a good day.

Simple quick question: is it possible to re-use a username or E-Mail adress from a user that has been deleted? For testing I am creating and deleting a lot of useraccounts and sites, then deleting them again, but I cannot Register a new username with same username or E-Mail like the deleted one.

Is there something like a "Cache" or table that I could flush in order to re-use same username or same E-Mail again?

Kind Regards,
Manuel