Apple's updated syncing service is a disappointment for PC users. Same for those other platform independant solutions that work well and sometimes better than mac originals. Apple Photos Duplicate Annihilator for Photos is available here.
I run the application regularly on the whole thing, and on parts of it where I suspect there might be some dupes, and it never fails.
If you wish to add more photos from other Events or Albums,. Plistedit Pro. From within iPhoto, you can manage your Flickr, Facebook, and MobileMe albums with the same ease that you've enjoyed on your Mac. Offer expires September 30, Extended offer! So now you can, if you wish, safely delete the files from your device.