I don't know if such a mod exists, but I wonder how do you imagine how something like this would work?
I mean, most games prevent this by requiring a login, so the player name is ensured to be unique as it resides in their database, however in a game such as this there's no sure automatic way to do so reliably.
At most, an unique ID could be assigned to a client, and then have a name bound to that ID.
But then from here you would have to assume a "first come first served" detection, where the first one to use a nickname would "own" such a name, unless you manually create or generate this ID and give it to the player directly to save it in some ini from theirs, to ensure it's them.
Also, if the player got to reinstall the game, or clean things up or so, this ID would be lost, and the player would no longer be able to use such a nickname until you intervened as an admin, to not mention that these could also be stolen in indirect but clever ways (probably wouldn't happen, but still...).
IP addresses and ranges could also be used, unless the impersonators you want to avoid are from around the same area as the "real" owners of such a nick.
Another solution would be to actually have user and password saved in your server, where a player would have to type at least once (to generate an unique ID) to get access to use it, which would solve the reinstall problem, and the IP range could be just added security, although it would probably be a bit overkill for this sort of thing.
What I mean with all of this is that, if such a mod exists, something along these lines is probably how it works, and it will never be very reliable, and although it may suffice in your case, I think you should understand what you might be getting if you find one.
PS.: I already used a nick in the past which collided with someone else's nick without me realizing it, and something like this would indeed have been useful.
I have ever since changed my UT in-game nick to something else that I was sure which doesn't collide with anyone, but anyone could indeed adopt a common enough nick to cause issues for others, this is one of the reasons central databases are used nowadays to store them.
