misses empty lines while checking and therefore finds less dupes Rated 4 out of 5 stars

nice idea, but fails on mails with differ in just two empty lines (CR or LF) at the bottom when saved and compared.
but a nice GUI (which misses a way to quickly change the settings for the dupecheck though).

This review is for a previous version of the add-on (0.3.7). 

That is not a failure...

When using the number of lines as a comparison criterion, such two messages are indeed not dupes of each; if you remove this from criterion they will probably be determined to be dupes.

Also, there is a quick way to change the dupe check settings: Add the menubutton to the toolbar.