Assegnate 5 su 5 stelle
This add-on works fine with TB60, I tested it, but you need to modify a little the code.
Uninstall the add from you TB
Download the XPI on the add-on : https://addons.thunderbird.net/thunderbird/downloads/latest/remove-duplicate-messages/addon-956-latest.xpi?src=dp-btn-primary
Open open the XPI folder with 7zip or other
Into the folder you will a file named "install.rdf", extract it and open it with Wordpad or equal
at th 11th line you will have "maxVersion>31.*"
change this by "maxVersion>60.*"
save the file, and put it into the XPI archive (replacing the old one)
install manualy from TB (the wheel in the top of the add page in TB)
Reboot and enjoy !
Note this tip works with a lot of old adds.