When Outlook downloads duplicate messages from the email server, there can be four possible reasons:
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Outlook Downloads Messages from the POP3 Server Twice
Outlook may download messages from a POP3 (Post Office Protocol 3) server again after emptying the Deleted Items folder, even if both the "Leave a copy of messages on the server" and "Remove from server when deleted from Deleted Items" options are enabled. This occurs due to a background synchronization happening simultaneously with the deletion from the server. Unfortunately, there is no workaround other than avoiding synchronization around the time the Deleted Items folder is emptied. This issue does not result in data loss, except for messages you intended to delete; simply delete the duplicate messages. -
Prompted for Password Leading to Duplicate Messages
If you log into Microsoft Outlook with a profile containing two POP3 accounts, you may be prompted for a password for one account, resulting in duplicate email messages. This can happen when both accounts point to the same POP3 server. To resolve this, turn off email reception for one of the POP3 accounts:- Go to Outlook > Tools > Send/Receive Settings > Click Define Send/Receive Groups.
- Select the group containing the two POP3 accounts, click on one of the accounts under Accounts, and clear the "Receive Mail Items" checkbox.
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POP3 Headers Are Duplicated
Pressing the F9 key to synchronize your Microsoft Exchange mailbox with your offline .ost file may lead to duplicate headers from your POP3 account. This occurs when the headers from the POP3 account are placed in the .ost file along with messages from the Exchange server. To resolve this, enable the "Download Complete Item including attachments" option in the POP3 account:- Go to Outlook > Tools > Send/Receive Settings > Click Define Send/Receive Groups.
- Select the group name, click Edit, select the POP3 account under Accounts, and click "Download Complete Item including attachments." Click OK and then Close.
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Antivirus Software
Some antivirus programs that scan email can cause this issue. If messages are large, the software might create a 'timeout' on the server before identifying the message as read, leading to repeated downloads. The quickest solution is to log in to the mail server and delete the problematic email message, or all messages, for a fresh start.
NOTE: Ensure you have downloaded all your messages before performing this action.