The User Name for our system is your email address, there cannot be multiple accounts with the same Email Address.
In your account settings, you set a Nick Name for your Community Profile, that is the name that shows in the forum. There is no restriction to make the forum nickname unique. Same as there is no restriction for the name to be unique in a phone book. I can look up multiple Jason Byrnes's in the white pages, but only one of them can have my email address.