Hi all, think I may have the answer.
Reading back through the posts, its clear that there is a common link between duplicating photos and tagging not 'sticking'.
In my case, I checked all the faces that would not accept tags, and sure enough they are the exact same pics that I have uploaded to facebook and tagged in facebook.
In iPhoto, the name appears along with the box containing the face, but crucially the name tag in the info pane is blank. According to iPhoto there is no name associated with the image.
I went into my facebook folder (my name under the WEB heading in the left hand pane) and deleted the tag in the photo, and sure enough the problem was resolved!
Hope this helps others too.
Thsi is a good bit of diagnostic work, in the absence of any information on how to turn on debug logging in iphoto.
This repeatable scenario and work around you describe should give the apple Iphoto development team a clear area for focus on to provide a fix. (yes i know eternal optimist)
I have seen this problem in some of my found Faces. I have also seen a number of other signs of Faces database corruption. FYI this is duscussed, with partial solutions, in this thread