If your Mac is signed into iCloud (and you have Contacts set to sync), you can go into the Address Book application and use the Card | Look for Duplicates menu bar option.
The only thing I don't like about that is that it tells you the number of duplicates it found, and gives you the option of merging duplicate cards, but it doesn't give you a preview of the data it found.
That being that, the feature has always worked well for me. 🙂