Rated 1 out of 5 stars

I suppose it might be a decent set of coding but it finds no dupes in my sent folder. Unchecked the selection in the dialog box. They're clearly there. Does not work.
Thanks for the reply Eyal. On my installation the dialog box looks nothing like the one depicted on the linked page. I'd post you a picture but I don't think this is the right forum for even this type of conversation. As I said earlier, I have unchecked Skip Special Folders (Sent ...)--there is no selection for a positive "search special folders", only a Skip selection to be unchecked--and left all other check boxes checked. I don't know why I get duplicated messages in my Sent folder but, regardless, there are two of everything so it seems the application should not report that there are none. I'm perplexed.

This review is for a previous version of the add-on (0.3.15). 

You need to make sure to set the appropriate comparison criteria. If you were to compare your sent messages by send time, obviously there would be no duplicates since you don't send different messages at the same time.

And again - you'll need to enable searching special folders.