Why do outlook duplicates occur & how to prevent

First, make sure you are not getting duplicates on the server in your mailbox. That will help determine if it is a server issue or an Outlook issue the mails which got downloaded more than once on your Outlook are duplicates sent by the sender or have been downloaded more than once by Outlook. To do so, simply view your email via Webmail. Next, double check the recipient addresses of the email. You may have been listed twice, or if you have two different email accounts going to the same inbox (such as a forwarded email) and someone sends you the same email to both address, duplicates will occur.

If neither of the above are the case, there are myriad reasons Outlook might be retrieving duplicates.
The reason for Outlook duplicate records could be that Outlook downloaded an email twice from one or more POP servers.

This could happen for multiple reasons, a background synchronization happened at the exact time you were emptying the Deleted Items folder, messages you intentionally leave on the mail server are downloaded again on the next Send/Receive, you have two or more alias accounts pointing at the same POP3 server, or other inconsistent occurrences.

To work around this issue, configure the POP3 account to deliver email messages to an Outlook data (.pst) file. Or, you can disable the Leave a copy of messages on the server option for the POP account.

Outlook 2007
To change the delivery location of messages for a POP3 account in Outlook 2007, follow these steps:
Click Tools –> Account Settings –> Select the POP3 account that you want to change –> Click Change Folder –> In the New E-Mail Delivery Location window, select the folder that you want to use –> Click OK to save your changes.

Outlook 2010
To change the delivery location of messages for a POP3 account in Outlook 2010, follow these steps:
Click File –> Account Settings –> Select Account Settings –> Select the POP3 account that you want to change –> Click Change Folder –> In the New E-mail Delivery Location window, select the folder that you want to use –> Click OK to save your changes.

To set the Leave a copy of messages on the server option in Outlook 2007, follow these steps:
Click Tools –> Click Email Accounts –> Click View or change existing email accounts –> Click Next –> Select/Click your POP3 email account and click Change –> Click More Settings –> On the Advanced Tab click to select the “Leave a copy of messages on the server” check box –> Click OK

To set the Leave a copy of messages on the server option in Outlook 2010, follow these steps:
Click File and Select your email address –> Click Account Settings –> Select/Click your POP3 email account and click Change –> Click More Settings – On the Advanced Tab click to select the “Leave a copy of messages on the server” check box –> Click OK

Re-installation or Reconfiguration of Outlook resulting results in duplicates. To prevent this problem, make sure you start Outlook with a clean .pst file after re-installation. Once the “new” email messages have been received, open your old .pst file and move only the new messages (not the old message “duplicates”) into the old .pst file. Then delete the new .pst file and set your default delivery to the original .pst file.
Sometimes your firewall and/or anti-virus software can also create Outlook duplicates in Outlook. In a Send/Receive cycle, Outlook sends a request to the mail server for mail messages. New emails not in Outlook are then downloaded and the connection is closed. However, if your firewall or anti-virus software interjects itself and causes something to fail at any point in this process (flagging a large message, etc.) the connection will not close properly. This means that any emails that were downloaded before the point of failure will be downloaded again on the next Send/Receive, thereby creating duplicate emails in your Outlook inbox.

If you think your firewall/virus scanner is causing Outlook duplicates, change your security settings to not interfere with Outlook. Outlook’s “in-house” security measures are tight enough and it does the work for you, so there is no need to worry.
Also do check your rules in Outlook as they could be set to send the same message to two or more places. If you have multiple rules setup to deal with incoming messages, there is a strong chance of Outlook creating duplicates in order to satisfy the two different rules.

For example, if you have a rule to send all emails from “ABCD” to a specific folder, and all emails with the subject line “Outlook” to go to the same folder, and you get an email from “ABCD” with “Outlook” in the subject line, technically both rules apply. Outlook will then move the message twice, creating a duplicate email that you will want to remove.

To prevent this problem, be sure to add the action “stop processing more rules” to the rule. This way, if a rule has already been applied to a message, no further action will be taken and duplicates will not be created to satisfy all rules.

There are some external tools or add-ons available online to download that can work as an add-on to Outlook to prevent this issue.

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