Active Directory duplication

Hi guys,
The forums have been great, I'm nearly ready to demo ServiceDesk to the rest of the department and then we can purchase. I have one last remaining problem.

This is what I'm trying to do:
1) Get the Requesters into the system using the "Import from Active Directory".
2) It works! Now, a few days later, I want to "Import from Active Directory" again - this is to retrieve new users since the first time. ServiceDesk says "Please Note: On Importing, Existing data will be overwritten and New data will be added." Good, this is what I want to happen!
3) It doesn't work. It does bring in the new users, but also duplicates all the original users, which haven't changed. So now, instead of 250 requesters, I have 500 users. In fact, every time I "Import from Active Directory", it doesn't remove the original requesters, it duplicates them again. Can this be fixed? Is there a quick way to remove all the requesters already in there?

Many thanks for your continued support,
Gavin