How to remove duplicate photos from iPhoto?
Use Duplicate Annihilator http://www.brattoo.com/propaganda/
There are several software that you can use to delete the duplicate photos from iPhoto. You can try Duplicate Cleaner For iPhoto, iPhoto Library Manager or Duplicate Annihilator. Personally, I recommend you Duplicate Cleaner For iPhoto because is free of charge, easy to use and doesn't consume many resources.