[CXdur]

This seems poorly coded to me if this is at all able to happen. I have no idea how you've made it so it's hard to give perfect advice, but the person above stated some very good points. What you're doing is simply a poor way of doing it and rather than looking for fixes to it you should make your clan tags appear another way.

Anyhow, assuming that your current way wasn't working correctly and that every player is saved in a database, you could always loop through online players when logging in to see if someone online has the same database user id. You could then just kick them. On phone, sorry if it's poorly written.