If you are using automatic scheduled send/recieve, try bumping the frequency up to 10 minutes. We did this along time ago and our dup emails dropped dramatically. It was something to do with the way outlook talks to the mail server, and having a scheduled send/recieve set to run within short periods of time was somehow causing outlook not to report to the mail server that it had recieved the emails, so of course the server would try and sent them again and again etc etc....
"Have you ever imagined a world with no hypothetical situations?