Since handles are not unique on its own, but pubkeys are, it's possible to have the same handles. However, this means you would need to trust who owns the domain to not assign jason@nostrplebs.com to some other user's pubkey.

If I were to turn the server off right now, no one can register that same NIP-05 ID until they buy the domain after it expires in a year. But as I said, I have no plans on letting either of these things happening :)