How to make usernames unique

[XFFA.IR Bot]

I launched my xenforo forum not too long ago. I've had on three occasions occasions I receive alerts asking to approve usernames that others have already. I would have figured this was a standard feature. I don't see anything in the US registration options setting to turn this on as a unique field.

Am I going crazy and this is a standard setting? If so, what are the admin alerts about that I am receiving?

If it is something I need to do, how can I modify the settings?