I had this problem with a computer running under Vista. When Outlook 2007 was restarted, it would download duplicated emails. The following is not the only reason this may happen, but in this particular case it was, and you might want to try it if you have tried everything else and hasn’t worked.

The reason in this particular case was the file outlook.srs –  a file that Outlook uses to keep track of received e-mails amongst other things -fails to flag downloaded emails as received. So when Outlook re-opens and looks for new email in the web server, it finds the old emails but thinks they are new, and downloads them again. For this to be true Outlook must have been set to leave a copy of the downloaded emails in the server.

To solve the problem, the following file needs to be deleted:

C:\Users\username\AppData\Roaming\Microsoft\Outlook\outlook.srs

Where username is your user name. Another way to access this file is to copy and paste the following string in the Windows Explorer address bar:

%USERPROFILE%\AppData\Roaming\Microsoft\Outlook

That will open the folder that contains the outlook.srs file.

If you need help getting this done or have other problems with your Outlook program, feel free to contact me.