Some AppRiver clients that use POP hosting are having issues with their Outlook email client downloading the same messages over again creating duplicates of each email in their client. Many times this is viewed as a server issue but research has indicated that the issue actually resides on the client side.
All versions of Outlook
Outlook stores a sync file on the client’s workstation that keeps track of which messages have been downloaded if the user opts to "keep a copy of the message on the server". On occasion that file becomes corrupt causing Outlook to lose track of which messages have already been downloaded. Microsoft states that the fix for this issue is to ensure Service Pack 2 and all other updates for MS Office are installed.
Other settings that may affect this behavior are:
1. Send/Receive time set too short
Don’t set this less than 5 minutes. If Outlook tries to Send/Receive while it is still working on the last Send/Receive it can cause duplicates
2. Try increasing the Server Timeout setting in Outlook.
• Select Tools>Accounts>Next>Change>More Settings>Advanced tab.
• Slide the Server Timeout slider to the right.
3. Check any mail rules to be sure they end in “Stop Processing”
4. Ensure only 1 instance of Outlook is running
5. Is there an alias email address for this user? Could the email be cc: to both?
6. Are there multiple POP accounts setup in Outlook?
Possible email is sent to both email accounts and they are both showing up in the Inbox
The link listed below is a MS Support article describing the issue in more detail.