PhotoDedupo

Easily find and delete duplicate images on iPhoto '09 (version 8.x) or later. You can customize the search criteria.

After search, you can use auto-marking to automatically mark duplicate photos for deletion to iPhoto's trash. Currently it has auto mark by date, title length, file size, image size, has RAW and rating.
PhotoDedupo can find both exact duplicates and similar(slightly modified) duplicates on iPhoto. If you have hidden photos and want them to be included on search, please unhide them first.
Note: PhotoDedupo can't search Aperture libraries and referenced images.
You can download free trial on http://www.photodedupo.com