Sometimes the sync process is going wrong somewhere along the line and creating duplicate lists of some contacts to build up. You need to take time to find out and delete the duplicated contacts by hand. AddressBook Cleaner is exactly such utility to help you find out all the contacts match the predefined fields, and mark the ones to be thrown, what you need to do is just run the application and perform simple clicks.
AddressBook Cleaner v1.163 MacOSX Incl Keyfilemaker-CORE
AddressBook duplicates cleaner clears duplicated contacts in Address Book on Mac.