Some orders are being duplicated in the system for some reason. I can't track it down. If this occurs to you, please let me know by telling me what page you landed on after checkout (success or cart). I think it's a problem with user sessions and I'm guessing that it only occurs with Internet Explorer (which never works correctly for any task). I'm really stymied on this one :(
Note that this doesn't affect your ability to place orders or download items. It just leaves you with two duplicate orders.