PhotoDedupo assists you in finding and deleting duplicate photo files in iPhoto. You can set the criteria of duplicates, whether it's the same size,content,name and/or date. PhotoDedupo uses byte to byte comparison to find duplicate content so the duplicates have 100% exact content (file's copies). Currently it's unable to find duplicate images on different format, size, etc.

The Auto Mark feature on PhotoDedupo is very handy and will save you a lot of time. You can automatically mark files by comparing date, title length or rating.

Requirements:
OS X 10.6-10.8
Photo '09 (version 8.x)+
Intel