I keep a recurring task in Omnifocus that reminds me to clean up my address book every six months. Previously I’ve done this by hand, and I’ve also used the built in
Look for Duplicates… command.
I think I first heard about Contacts Cleaner on the Mac Power User podcast, but I finally gave it a try. It worked incredibly well, fixing a ton of mistakes that I’ve been living with for a decade (in some cases). It shows you a preview of the way it will clean up a contact:
It sounds like I’m not alone in recommending it: