The link below will take you to an article from Microsoft's discussion forum which explains the issue of duplicating emails which users of Outlook 2016 are currently reporting:

In the meantime, it is also possible to revert to the previous Outlook 2016 version, by following the below instructions:

How to revert to prior version:

         a) From Outlook 2016 menu

                File --> Office Account --> Update Options --> Disable updates

         b) Set a reminder on your calendar to re-enable updates ~April 1

         c) open as an administrator

               for W8/W10 - right-click Start button --> select Command Prompt (Admin)

               for W7 - type CMD in Start search box --> right click --> Run as adminsitrator

         d) paste/run the following in the command prompt - 2 lines, each followed by <return>:

             cd C:\Program Files\Common Files\microsoft shared\ClickToRun

             officec2rclient.exe /update user updatetoversion=16.0.6366.2068

As a side note, if you slightly uncomfortable carrying out the above instructions, it will be best to consult with your technician.