Rated 5 out of 5 stars

I don't know what I would do without this!
The only improvement that would be really good is the option to "Keep" (prioritise) messages that have been replied to. That way they could remain part of a long discussion thread & not break in the middle if I've chosen the wrong one.
How do I give you money? (No PayPal link on the Add-ons page).

This review is for a previous version of the add-on (0.3.16). 

Rated 5 out of 5 stars

I use this because it helps & the idea is right, but I have a lot of duplicates that it doesn't find.
I cannot find any differences but there must be some, such as a few seconds difference in Received time(?) but there's no way to set any "ignore" criteria like in C-Cleaner's Duplicate (Files) Finder.

Edit: I can't reply to your reply so I will do so here -
Thanks. I was looking for the "Add-on settings" in Tools > "Remove Duplicates..." where all I got was a short list of messages.
There's no "button" in the toolbar from what I can see?
But I've just found "Options" in the Extensions > More link where I can set criteria.

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

You can use the add-on settings dialog to choose which criteria to use for the comparison. Also, the toolbar button has an integrated drop-down menu from which you can select the criteria.