Users who have Office 365 will have duplicate emails if they use a POP account.
First, disable Office updates. Inside Outlook, go to File – Office Account – Update Options (dropdown) – Disable Updates.
Close out of all Office applications that are open.
Now open an elevated command prompt (Search – cmd – CTRL+SHIFT+Enter) and run the following:
C:\Program Files\Common Files\microsoft shared\ClickToRun\OfficeC2RClient.exe” /update user updatetoversion=16.0.6366.2068
NOTE: This reverts you to the February build, you will need to re-enable updates in the future once MS comes up with a fix for this.