The requirement for unique user names is a little strange, I was putting together a small internal tool recently and after a bit of thought decided to use an opaque internal id for users and let the users pick and change their name and secret at will.

I think for a larger public service it would make sense to expose some sort of internal id(or hash of it. What bob am I talking to?. but people share the same name all the time it is strange that we can't in our online communities.