I would like to suggest you this third party tool for Remove Outlook Duplicates to effectively remove all kinds of duplicates items in PST file. The software is also well works on all MS Outlook and Windows platform. It removes whole duplicates in your file very easily and without any error.
Clean up Conversation will delete and move the duplicate emails that are in the conversation
Clean Up Folder will remove the duplicate emails from any particular folder
Clean Up Folders and Subfolders will clean up the duplicate folders and subfolders.