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If you are getting duplicate emails on your computer in your Outlook Express, it could be that there is a corrupt email message in your inbox on the mail server. To delete the corupt email, simply:

1. Login to your mail server
   http://mail.comentum.com/mail/
   - Enter your user name (your email address)
   - Enter your password
2. Delete the corrupt/damaged message that is creating the duplicates
   (You may want to delete all messages on the server, after you have downloaded them to your Outlook.)

Possible Causes of Duplicate Emails on Outlook Express

Problem: The "Leave Messages on the Server" box has been checked on your Outlook Express account settings.
Fix: Uncheck the "Leave Messages on the Server" box. To do this, click on Tools>Accounts. You will see a box open (named Internet Accounts), click "Mail" tab, click on the mail account, click on Properties>Advanced. Remove the check in the box "Leave a copy of messages on server."

(The following information is from Microsoft Help and Support)
Problem: Outlook Downloads Messages from a POP3 Server Twice
Outlook downloads messages from a Post Office Protocol 3 (POP3) server again after it empties the Deleted Items folder even if you have both the Leave a copy of messages on the server and the Remove from server when deleted from Deleted Items options enabled on the POP3 account.
Fix: There is not an available workaround for this problem. . . Simply delete the duplicate messages.

Advanced Configurations:
How to setup email forwarding
How to create an Auto Responder
How to turn on Spam Filter

REFERENCE
http://www.comentum.com/outlook-issues.html