How to protect from differrent users create same other user name?

How to protect from differrent users create same other user charector name?

Please, very need advised coz, On War Time. The other person create new user with same name charector with the person who's support to put card in computer to win a war.

But, the other person with different user but, they created same charector's name with the person who's support to put card login, the charector who's support to put card lock off from server!

Any advised for this bro! BIG THANKS