Usernames are not unique, however you can get yourself a nostr address (eg. https://nostrcheck.me will give you a free one

A Nostr Address looks like an email address (eg. mine is bitcoinbarry@nostrcheck.me)

This takes advantage of the existing uniqueness of DNS names and your username is unique to that domain name.

Of course someone could buy n0strcheck.me and duplicate my account and use bitcoinbarry@n0strcheck.me as their nostr address, so people still need to stay vigilant, but it still helps.

Also, if you own your own website, then you can use @ which can help give a little more credibility like how having a Gmail account is cool, but having a custom email address is considered more professional.

I suspect celebrities and influencers will use their own websites.