In the FAQ, we strongly encourage people to use their real names, but ultimately this is not something we can enforce. Identity is very difficult to prove on the internet, and I don't think people should be forced to tie their identity to their MO user. For what it's worth, usernames are not unique, so nobody will ever be prevented from using their real name if somebody else is using it. If two users are using the same name (e.g. I see we have six users whose entire username is "David"), you'll just have to tell them apart by their avatars and user numbers. If some user is deliberately trying to cause confusion by imitating another user, I'm happy to issue a temporary suspension and tell them to cut it out. Somebody using the name "Grothendieck" is mildly annoying, but not an actual source of confusion.