We have a number of duplicate nicknames, which shouldn't happen. http://pastebin.mozilla.org/670801 We should try to figure out how this happened in advance of actually needing nicknames to be unique for the forums.
Unique nicknames are now enforced by the database. We can investigate this sometime but it's no longer a 5.1 thing.
(In reply to comment #1) > Unique nicknames are now enforced by the database. that's enough --> fixed.