Rated 4 out of 5 stars

nice idea, works ok.

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

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.