brilliant (just some German missing) Rated 5 out of 5 stars

You did a great job - thanks! The add-on works just fine and very quick for my some hundred mails.

Yet it took me some time to find out (by reading other comments) that I need to remove the checkmark on "skip special folders". Before I did that I tried hard to remove duplicates on my imap account (using special folders) with no success and also no message from the add-on except "Looking for duplicates".
I would prefer that you choose the default as "skip special folders" = NO.

By the way the German translation lacks some strings in "...extensions\{a300a000-5e21-4ee0-a115-9ec8f4eaa92b}\chrome\removedupes.jar\locale\de-DE\removedupes\removedupes-prefs.dtd"

<!ENTITY message_comparison.time_comparison_resolution.description "Vergleich von Zeiten mit:">
<!ENTITY message_comparison.time_comparison_resolution.seconds.label "Sekunden">
<!ENTITY message_comparison.time_comparison_resolution.minutes.label "Minuten">
<!ENTITY message_comparison.time_comparison_resolution.hours.label "Stunden">
<!ENTITY "Tagen">
<!ENTITY message_comparison.time_comparison_resolution.month.label "Monaten">
<!ENTITY message_comparison.time_comparison_resolution.year.label "Jahren">

Without them the settings dialog will fail.
There are also some other strings in the file that are not yet translated to German, but still English.

Thank you for your work.

This review is for a previous version of the add-on (0.3.4). 

Fixed in 0.3.5

The locale issue is fixed in 0.3.5 which you can download and which will become the released version very very soon.