Seems that this problem isn't a delivery problem. The inbox file only gets one copy of the mail. However I have found that all 3 clients are using Microshaft Outcrook Distress or Outcrook XP.
2 of these people have it set to leave a few days of mail on the box. There is a inbox.pop and inbox file. What's interesting is they say it takes a few minutes and that's when they see the second copy. It also seems that it's only from local sent mail. I was thinking it may have something to do with the way the pop server calculates the time stamp on the email and messes up when it compares the last email poped.