Last month, Outlook threw a wobbly, in that it somehow downloaded a second duplicate copy of every single email still stored on Plusnet's mail server. Thousands of them, how I just don't know.
For ease of use - I have my Inbox divided into multiple folders, with mail sorted into the folders, dependent on the source of the mail. Since then, I have been wondering just how to sort out that mess of duplicates. Yesterday I found it - left click on the folder and a menu pops up, one of the items is Cleanup. That deletes all of the duplicates in one click.