Seems like to me there is no possible solution because:
A) You can't code for "Pre-Login"
Once they have logged in, the damage is done?
Nevertheless, obviously you will need to keep a variable of nNameCount in the database to know how many names exist and each time a new name is created, that is indeed original, you will need to set that nNameCount +1...
Essentially you will need a unique Name Identifier for each entry in the database..
string sBase = "ORIGINAL_NAME_"
string sCount = IntToString(nNameCount);
string sNew = sBase + sCount;
Store it into the database...
Every time a NEW player logs in, check to see if their name is original by looping through all of the entries in the database to see if sPlayersName exist already, if so, boot player... (Obviously the PC won't know why they can't log in.)
I suspect it's a good way to run off potential players though, no doubt...