Duplicate Emails

I have 20+ users at one site where one user complains of receiving 2 and 3 copies of each email while using Outlook. This happens on an intermittant basis 2 to 3 times a week. So far, none of the other site users report the problem and they’re all configured the same. He’s runs Outlook 2K configured to use Internet Email, a PST file, and a PAB. His PST file is currently 176M in size. I run Echange 5.5 SP4 at my site.
I’ve checked the status of the “Leave a copy on the server” switch and it’s off. I’ve only seen this situation happen with the switch turned on. Any one have an ideas?