also keep in mind a common outlook/outlook express problem.
outlook and outlook express will reload all the mail sitting at the server account anytime there is a change in the server settings tab.
if you go and make a change in the smtp or pop server name, user, pass, and all that, then outlook sees it as a new config and downloads everything, instead of just what you last left off with.
also the settings in outlook such as 'leave mail on server' will be part of this issue. if you dont need that capability, then you can turn it off and this duplication will not occur.