Yeah, after importing contacts back and forth between my android phone and Thunderbird I’m in the same boat. Trying to avoid manually selecting the 400 or so duplicates to delete them… The duplicates aren’t visibly listed in my phone’s app but when I export a .vcf file from it and open it in Thunderbird or Gnome Contacts they are. I’m surprised that my desktop apps don’t have something inbuilt to deal with this.