Theres also a setting in gmail itself which may help you as it controls how you can get mail from the server. If you open gmail in the browser, log in and go to settings you can now tell the gmail that when accessed by pop it will only download mails from that point onwards - this should help minimize the amount of mails your client receives.

I never had the problem with it receiving the same mail each time in the built in mail client on OS2006 or OS2007HE but I do find that claws is much nicer and feature rich - has a gecko plugin which will correctly render html messages and an RSS reader plugin which also works great.