Entourage re-receiving ENTIRE email list

Every month or two, my entourage decides that it wants to see all 3,500 emails from all three of my accounts again and RE-receives them. It takes over an hour and it just downloads a duplicate copy of every email message in my inbox from all three of my accounts - 3,500 of them! How do I stop this? Is it some schedule I am not seeing?

If you're using POP3 email, you need to change your preferences so that Entourage does not leave leave a copy of the messages on the server.

Entourage > Account Settings > (Choose your account) > Options.
Uncheck the box that says, "Leave a copy of message on server".

You'll have to do this once for each individual email account.