I am not sure what you mean but if you are trying to only have a unique set of usernames in that table you would do something like this in SQL:
IF NOT EXISTS(SELECT username FROM table where username = @username)
INSERT INTO table (username) values(@username)
--Stole this from a moderator
I will only tell you how to do it, not do it for you.
Unless, of course, you want to hire me to do work for you.