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A bug within Outlook 2016 causes the POP client to re-download messages causing duplicates to be seen.

There is a known issue within Outlook 2016 which when setup with a POP account causes e-mails to be re-downloaded from the server. This in turn causes duplicates to appear in the inbox within this client.

Until such time as Microsoft release a fix for this issue please follow the steps below to revert to an earlier version of Office 2016 to rectify the behaviour.

• In Outlook, click on the File tab > Office Account > Update Options > Disable Updates.

Note: Set a reminder on your Outlook calendar to enable updates on April 1, by clicking Enable Updates under Update Options.

• Close all office applications.
• Open a command prompt. Type the following command, and then press Enter.
• "C:\Program Files\Common Files\Microsoft shared\ClickToRun\officec2rclient.exe" /update user updatetoversion=16.0.6366.2068

Note You can replace the version number in the command if you prefer to revert to a different earlier build, such as shown in the following example.

"C:\Program Files\Common Files\Microsoft shared\ClickToRun\officec2rclient.exe" /update user updatetoversion=16.0.6366.2062

For more information about the version numbers of update branches for Office 365 clients, go to the following TechNet article: Version numbers of update branches for Office 365 clients
• After the update is downloaded, if any Office applications are open, you are prompted that they need to close. Save your work and click Continue to close the applications.
• When the update is finished applying, start Outlook again.

This program (officec2rclient.exe) will run and update/downgrade the current version of all office applications to the last known "good" one. These steps will stop a new duplicated email but will not remove any duplicate emails already downloaded. These must be manually deleted from within the Outlook application.