Rated 3 out of 5 stars

To stop double notifications, disable the extension, restart Thunderbird; enable the extension, restart Thunderbird. Apparently you need to disable/enable the extension once post installation. But it does reduce the growl notifications to only one per message when you do so. I do like the basic functionality of the extension, but it is a shame that the developers don't seem to have enough interest in the extension to fix that basic problem, or to address some of the suggestions to improve it.