That can only be achieved in a centralised service.
In nostr the keypair (public and private keys) is the only guaranteed unique identifier.
Discussion
I'm still new here, but the mere existence of the concept "handle" feels kind of redundant. Seems like pubkey + display name would be enough.
I think it's fine for now, bit it'll become contentious when public figures join and they attract imitators. When that happens, integrations with other decentralised identity management systems (KILT protocol etc) will become more important, IMO.