At first this app seemed to not work. I get dupes intentionally because I have gmail also reading my main mail account, partly to have a backup on another server, partly as a defense again false positive spam blocking. By changing the default setting to be only Author, Message ID, Sent Time, and Subject, it properly removes my dupes. Unlike another app for dupe reduction, this app does not leave my keyboard delete key disabled after running it. Your mileage may vary.
This review is for a previous version of the add-on (0.3.15).