If you want a perfect solution for remover duplicate items in outlook then i would suggest you to use any third party software. All the software are brilliant and
effective to remove duplicate emails but i highly recommend you this Outlook Duplicate Remover Tool is helpful and useful software to remove all the duplicate emails from mailboxes and optimize the performance of MS Outlook.
This application also eliminates the .pst files from the folders with multiple duplicate Outlook PST files. It allows a user to deduplicate emails, contacts, calendars, notes, tasks, and journals.
The can also view the .pst files with its attributes including the file name, file path, the number of duplicates found, their name and the status of files.
A status report is also generated by the program with the current status of each file and the completion status is displayed as the number of items proceeds.
Read more info:- https://www.filesrecoverytools.com/outlook-duplicate-remover.html