Not finding duplicates in Sent Items folder (Skip has been unchecked)

[msitekkie]

Having found numerous duplicate items in my sent folder as a result of the common IMAP issue, I attempted to use the Remove Duplicate Messages add-in, but it did not find any duplicates in Sent Items, even though I had unticked the Skip option in Settings.
I also tried restarting the mail client and searching again. I tried it at the top level and also with Sent Items open.