Sorry  for the long reply time.

If you are proficient in perl you can do exactly what you asked by modifying the code in one of these scripts.

 GO HERE

Let us know how it goes

Otherwise, you can use sessions to not allow dupes, though that can get a bit tricky.  Another way is to use a database table, have a record inserted on login, into a field that doesn't allow dupes.  But then you would have to either make sure users log out, or have a javascript call up some code that deletes the record when the window is closed...this could be contained in a session also.