If it's anything like Google's, they're single use, and you can generate new ones or revoke old ones.
They have some "device passwords" that do that as well (only works for PS3, PSP, Vita, I think)--these are one-time only passwords that can then be revoked. But the backup codes are just that, backup codes. But yeah, maybe once they're used, the screen displays new ones in their place. Maybe I'll have to try it and see.