I played a lot with the settings and unfortunately I found still a lot mails as duplicate. They were the same, not attachements. Quite easy to find them even by eye. No idea why the settings don't work.
Then I tried the xpi-addin from Thorsten (which is warned as not compatible to current Thunderbird version). It serves more information and it's much more precise - AND it found all duplicates. Unfortunately you cannot resize the windows.
So my recommendation is - ask for the code of Thorsten and recode it for current thunderbird versions.
This review is for a previous version of the add-on (0.3.16).