Ok, to clear things up when you create an account, you will create a unique username that is different to everyone else’s and you can not change it.
However, on the forums we all have a Display name which can be changed and can be the same as someone else’s. These two toms share the same display name but they both have different usernames to log into the forums.
If I wanted to, I could change everyone’s display name to Potato, but all your usernames you log in with will still be the same when you first created your account. Just everyone will see your display name as Potato instead of your username.
Username is different to display name.
Hope that clears this up.