When sending and receiving mail in outlook, messages are downloaded two times or more from the server resulting in duplicate messages in the outbox.  There are several possible causes for this behavior.

First, check this Microsoft Knowledge Base article and check that your software is up to date.  http://support.microsoft.com/kb/885870

You should also make sure that you are not receiving duplicate emails due to other accounts forwarding the same email into your account.  If you have questions about forwards and alias on your account, please contact us and we can verify your settings. We have also seen multiple accounts set up in Outlook that are set to check the same account on the server thus creating duplicates, make sure that if you have set up multiple accounts in Outlook that you are not pulling the same message twice.

Another possiblity is that there are messages stuck in your outbox.  When you have messages stuck in your outbox, Outlook cannot complete it's send and receive process correctly which may result in duplicate messages being downloaded.  Delete or move the messages in the outbox and restart Outlook to see if this fixes the problem.

Another thing to look for is how many instances of outlook you have running.  It's possible for Outlook to have two instances running which will result in both instances downloading messages from the same account and creating duplicates emails in the inbox.  To check this, close down Outlook and then open up your task manager ( CTRL+ALT+DEL , click "Task Manager") Look under the "Applications" tab for "Microsoft Outlook" and under the "Processes" tab for "OUTLOOK.EXE".  If either of those show up select them, and press "End Task" or "End Process".  It would be a good idea to reboot your computer afterwards.