A number of mail programs will do that if you choose to "Leave Mail on Server". Check all your accounts to confirm that such an option is not selected.
I believe the path is listed below:
Tools > Accounts > your email account > Properties > Advanced > Delivery...
UNCHEC "Leave copy of message on the server"
Hope this helps.
Grif
I'm running a desktop with Vista Ultimate and a laptop with XP Pro. The Vista computer uses Windows Mail and the XP Pro uses Windows Live Beta. I used Comcast.net for my email.
It doesn't matter which computer I access my mail from using the above mail programs. For example, say I have 10 emails on my laptop that I read and either save or delete.
The next time I log on to my desktop and check my mail, I get the same 10 messages again which garbage up my mail account and I have to go through the delete process again.
This really becomes a pain when I'm on a trip and I access several emails a day because of course, when I come home, all those emails will once again download. Now there may be 100 or more. I'm not sure, but I don't seem to remember this problem when I was using Outlook Express on my laptop. I also don't think I have this problem IF I access my mail through comcast.net rather than using the above 2 MS mail programs.
Does anyone know of some setting that will NOT download emails that have already been looked at on another computer?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic