Rated 3 out of 5 stars

le mlissner ar DFómh. 9, 2009 · buan-nasc

This is a pretty good add-on, but since it's not functioning for TB3 yet, I wrote up a how-to so that people can do it another way. It's still pretty efficient all in all:
http://michaeljaylissner.com/blog/de-dup-thunderbird-address-book-contacts