There is a common issue with Microsoft Outlook if you elect to save copies of your email messages on our shared mail server while utilizing POP3 as your account type.
Note: This is a known issue with Outlook that Microsoft has not yet repaired, and it is not an issue with our mail servers.
If you are using a POP3 account and attempt to save an email on the server (found under More Settings > Advanced Tab > Delivery), Outlook will attempt to download the email and not mark it as downloaded off the mail server. When Outlook checks for mail next time on your mail server, it will download a duplicate of this email. This will happen repeatedly until Outlook marks the message as downloaded on the server.
Until Microsoft fixes this issue, the only workaround is to change your Outlook to connect with IMAP if you would like to keep a copy of your emails on the mail server.
To receive your emails via IMAP instead of POP3, read Article #1547 which explains how to setup your email in Outlook 2007 or Article #1085 for Outlook 97, 2000, and 2003.