So for what happened with me not repeating, the only allowed characters in username should be the letters from A to Z and numbers from 0 to 9, probably a few symbol exceptions such as ©® and the one that is "TM" that I can't seem to find.
I used this letter: о (not a normal o) to make my username Carbon and this one: е (not either normal e) to make it be Glen. You can easily find a few more in the Character Map of Windows and you can look in other alphabets.