I am using "Fisherman" as my nick on my forum. Yesterday somebody has registered as "fisherman" the difference is only the letter "f" not written in capital, is this something that I have miss configured in my setting. I know that this is not a bug ı guess somethink that I've done wrong with settings but I can not find what it is.
Are you sure there weren't any dodgy (maybe invisible) characters in the username? MyBB should never allow duplicate usernames, even if the case is different.
BTW, are your users allowed to change their own names after registering? The name-changing script doesn't do as much validation as the registration script...
You won't get away trying to steal a username with changing nicks after registering either.