Tagging Faces in Aperture 3 and syncing with Facebook tags
If a face is already duplicated (i.e., there're two faces of the same person), you can pick up (from the faces cockboard page) the 2nd face and drop into the 1st. Aperture or iPhoto will merge these 2 faces into 1.

If a face isn't already duplicated and you're trying to prevent duplication, prior to 10.7 Lion, you set the email address of the entry in Address Book to that in Facebook. Apparently, this is the field that Aperture/iPhoto uses to match up Facebook &amp; Address Book.

For 10.7 Lion, Address Book has introduced a field called Profile->Facebook. By all looks, this looks to be the new field for matching, but Lion is too new to have confirmation from Google of my suspicion.