In my UBB.threads 6.0.1 i had the same problem as you. The problem was, if using unicode characters, the browser translated it into real digits.

Instead of register with "ABC" you could use
Code
  "ABC",
so it looked like there were 2 users with the username "ABC"

Try add this code to adduser.php
Code
$Username = html_entity_decode($Username);
       $Username = str_replace("&","",$Username);
       $Username = str_replace("#","",$Username);
       $Username = str_replace(";","",$Username);
       $Username = str_replace("%","",$Username);

trosfrihed.dk
Custom Ubb 7.5.2