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It does something but doesn't seem to accomplish anything on Thunderbird 68.2.2. Only the right-click menu item shows, but there are no preferences. When I select it from the right-click menu, the result is a popup that says "Could not find any duplicate messages in the selected folders.".

Edit 1:
The other clones of RD plugin in earlier versions of TB did find duplicates in the same circumstances. Also the preferences do NOT appear in Thunderbird Menu > Add-ons and Tools > Add-on Preferences
If that helps, I am using TB for Mac.

Edit 2:
The add-on preferences don't show neither in Add-on Preferences, nor in the Add-ons pane.
Also in my config there are leftovers from the original add-on from the time it was using it with an earlier version of TB, so in fact I do have saved preferences. These are not taken into count.

Last but not least, thanks for trying to help.

Questa recensione riguarda una versione precedente (1.5) del componente aggiuntivo. 

So you have no duplicates, the preferences are in Thunderbird Menu > Add-ons and Tools > Add-on Preferences.

Edit:
The prefs are in 2 places:
Thunderbird Menu > Add-ons
Menu Bar > Tools > Add-on Preferences

If you can't change the prefs, you don't know what you're comparing.
I don't have Mac right now, so I can't test it myself.