Spanning Sync is coming out with a new app soon aimed at 'cleaning up' our messy contacts and calendar data. Its available as public beta right now over here. I must say that the intelligence of the suite is impressive! Here is a quick look at the Contacts Cleaner application finding several 'problems' with my Address Book data - entries without surnames and email address which have been duplicated across contacts.
Spanning Sync states that the suite will find duplicate iCal events, Address Book cards, and as displayed above, duplicate entries within the vCards themselves. This will be a utility to adopt immediately for every Mac user who syncs.