Why does my Outlook keep receiving duplicate messages?
There are a couple of situations which may cause Outlook to download duplicate messages:

1. Outlook Downloads Messages from a POP3 Server Twice
http://support.microsoft.com/default.aspx?scid=kb;en-us;292249

2. Mail Retrieval Problems Occur When You Receive POP Mail
http://support.microsoft.com/default.aspx?scid=kb;en-us;317945

3. Password Prompt, Duplicate Messages Appear If Outlook Profile Has Two POP3 Accounts
http://support.microsoft.com/default.aspx?scid=kb;en-us;284404&Product=ol2002

4. POP3 Headers Are Duplicated
http://support.microsoft.com/default.aspx?scid=kb;en-us;289942&Product=ol2002

5. You are Using Anti-Virus Software
Many of the anti-virus programs that scan email can cause this issue as well. When the messages are large, the software causes a ‘timeout’ to the server before it sends back a flag that id’s the message as being read. As a result you receive the message over and over.

How can I stop this situation?
The easiest and fastest solution to this problem is to log-in to the mail server and delete your mail message that is causing the problem – or all of them for a fresh start. (NOTE: make sure that you have downloaded your messages first) Follow these steps to delete your messages from the server:

1. Go to scarletts-web.com and login to check your mail

2. Enter your entire EMAIL ADDRESS (your user name) and then your PASSWORD

3. If your mail is looping one message over and over then simply delete that one message
Other things to prevent duplicate emails:
1. Download and install the latest Microsoft Office XP Service Pack and
Patches. This is most easily done by accessing the link to Office Update from, http://windowsupdate.microsoft.com

2. Increase your server timeout settings to 10 minutes:
    Tools| E-mail accounts
    View or change existing e-mail accounts
    Select the account corresponding to your ITS UNIX account
    Click the Change button.
    E-mail Accounts
    More Settings
    Internet E-mail Settings
    Outgoing Server
    My outgoing server (SMTP) requires authentication
    Use same settings as my incoming mail server
    Advanced
    Outgoing server (SMTP): 2525
    This server requires a secure connection (SSL)
    Server Timeouts (Increase from 1 minute to 10 minutes)

3. Empty the Deleted Items folder only when Outlook is not checking e-mail in the background.

4. Configure Outlook so as not to check e-mail more frequently than ten minutes.

5. Configure Outlook to download complete Item including attachments.
Tools| Send/Receive settings
Define Send/Receive groups.
Click to select the group name, and then click Edit.
Under Accounts, click to select the POP3 account.
Download complete Item including attachments.-courtesy of Bernard Kohan