hadi on Nostr: When you're creating an account on Nostr and generating a npub and nsec, it's sent ...

When you're creating an account on Nostr and generating a npub and nsec, it's sent only on the relays that the client is using right?
So what happens if another person creates an account on others relay and got the same nsec? I mean, it's not likely to happen, almost impossible, but just want to know