An good post discussing this issue:
Other things you can try:
To stop receiving duplicate e-mails, check the following:
1.Check the Rules that you have configured to be sure that copies are not being made to the Inbox,
2.Ensure that all your rules end in "Stop Processing" actions.
3.Check the windows task manager to ensure that there is only one instance of Outlook running. If they are two instances of outlook.exe running, it may produce two messages arriving at the same time.
4.Delete or remove all your e-mail accounts and recreate them.
I5.f your running Outlook XP, ensure that you have service pack 2 or later. If you need to upgrade, just go to Microsoft Product Updates.
6.If you have two different POP3 e-mail accounts set up in your outlook, then you might receive duplicate e-mails. Check this article on Microsoft's website which explains the problem.
To prevent sending duplicate e-mails, check the following:
The reason for your PC sending multiple email messages might due to an anti-virus that is configured to check Outlook emails. When the Anti-Virus scans your outgoing emails, it might send it out twice. You can either change the settings on your anti-virus or refer your anti-virus manual for other alternatives.