So I had exactly this issue, in Aperture I had a single face that showed up as two in the iTunes sync panel. I carried out Lex's suggestion above which worked a treat (there were two versions of the name in the auto-complete dropdown though they looked identical to me)
EXCEPT:
In iTunes I now have a single representation of the face as opposed to 2 in BUT the number of photos indicated in iTunes is about double that I actually have / show against that face in Aperture. So somewhere in the faces DB there is still that duplicate name or other corruption that is somehow showing photos tagged with it also..
How can I remover that face completely from the Aperture DB - I don't mind going through and reidentifying relevant [photos, there aren't too many? I have already tried all of the Aperture first aid options to no avail.
thanks