Don't capitalize every letter in a sentence. It makes it painful to read.

What you're going to have to do is store all current names in a database of some sort. A simple list or savefile or something like that will suffice, though there are certainly many alternative methods. All you have to do is, upon an attempt to set a name, you check the database and see if the name already exists. If it does, you force them to choose a new name. Otherwise, add the new name to the list and set their name to it.