Since Google Contacts disallows multiple contacts with the same email address, duplicates usually aren't an issue. But contacts without an email address can become duplicated under some circumstances, so we've just enabled a server-side feature that should help prevent that from happening.
If you have duplicate contacts in Google but not in Address Book, and if Address Book has all of your contacts, you can perform a server reset (Advanced > Replace Data on Google > Contacts) to get rid of the dupes. Note that this will remove any group information or other "extra" information from your Google contacts that isn't stored in Address Book.
