As far as I noticed, despite of users specifying “nickname” during registration process, they get names based on their email’s localpart (before the @). And, in case of duplication, some integer is added to the end of nickname (like for me).
At the same time, I’m pretty sure, users will be happy to have their own nicks (which they specified during registration), and not that autogenerated thing.
So, I suggesting you to enable “Allow username changes” under “General options” in user registration settings.
AFAIR, phpBB (forum engine) will track their uniqueness itself.