I am not sure when it is happening. To try to get rid if the 5,000 odd photos, I deleted all my photos and went back to 'original' library and imported that again into iPhoto, there was 311 photos. When I then went into preview and the photo library they had increased to 926 - the extra photos were all 'face photos' of the original photos I imported! (by the way I imported the photos straight into iPhoto)