Rated 4 out of 5 stars

1. The documentation needs a lot of improvement. I had to search reviews to discover features. Getting to utility options was not obvious (For example: I had no idea the Sent folder was disabled until a review sent me to the Options). Anyway...
The program worked fine for other folders but the duplicates were initially left in place on the Sent folder despite a whole search and dialog being generated. I told it to Delete Selected but nothing happened that I could see. There was no indication I could see of anything like a count down happening which would have told me to wait. I gave up, and restarted TB when I discovered to my amazement that the duplicates were in fact gone.
Win 10 64 bit, Thunderbird 45.5.0, Most recent Add-on as of November 2016. TB says there were 21,000+ messages in Sent but the Add-on reported a few hundred fewer. This made me apprehensive though it may not mean anything.

This review is for a previous version of the add-on (0.3.12). 

Could you please file these bugs, separately, on www.mozdev.org/bugs ?

As for the documentation, I guess it could be clarified more, or I should possibly indicate in the dialog that special folders were skipped.