Symptoms

When downloading email into Outlook Express there are duplicates of each message.

Cause

This can be caused by a few different reasons. One reason is because ‘leave messages on server’ is checked the account settings of Outlook Express.
The other possibility could be the file named Pop3uidl.dbx is damaged.

Resolution

To remove the option ‘leave messages on server’ click on Tools, Accounts, a box will open up labeled Internet Accounts, be sure you are on the tab labeled ‘Mail,’ highlight the mail account, next click on properties. Once you click on properties, you will click on the Advanced tab. At the bottom of the Advanced page you will see a section labeled ‘Delivery.’ If there is a check mark in the box next to ‘Leave a copy of messages on server,’ remove that check mark. If there is not a checkmark in the box, proceed to the next resolution.

To fix the problem, simply delete the "Pop3uidl.dbx" file, do a search/find for "pop3uidl.dbx". The next time you check for mail, all the messages on the server will be downloaded again, but a new "Pop3uidl.dbx" will be created that should work properly on subsequent mail checks.