How to delete duplicate messages in Outlook 2016
Open Outlook 2016
Select the folder that contains duplicated emails.
Go to Home menu then click the Clean Up option available on the ribbon.
Choose any one option as per the reason for the occurrence of duplicate items.
Select Clean Up Conversation from the drop down options that appear. This will delete and move duplicate email messages existing in the conversation to Deleted Items folder.
Clean Up Folder option will allow you to delete duplicate email messages from a particular folder.
Clean Up Folder and Subfolders will clean up folders as well as subfolders.
Right-click the Deleted Items folder and select the Empty folder option to delete the duplicate email messages permanently.