Are they identical datestamps? or are they a few seconds (or more) apart?
Also, are you using POP?
If the former, it's because your client is not sending the DELT command to the mailserver after each transaction, and it's re-downloading the mail item every time it checks the server for mail. If so, i'd check whether your client is set to 'leave mail on server' which is known to screw up a bit sometimes. The solution for that is to either turn that feature off so you download the entirety of your POP mailbox to your PC, or failing that, ask your ISP to delete and recreate your mailbox (a bit drastic but it does the job!)
If the latter, it's someone else's issue and you should shout at your ISP anyway.