This typically happens when someone has copied someone else's install or user.ini. Or downloaded a config file from the internet. The client ID is uniquely generated when you first join a server. You can fix this simply by generating a new ID by following these steps:
Make sure UT99 is closed.
Go to the system folder in your unrealtournament install.
Find the file user.ini and open it with a text editor like notepad.
Find the section [NexgenCC.GeneralConfig] and delete it.
Save the file and reconnect to the server.
Once this is done the server will generate you a new client ID and you should be all set.