PLEASE NOTE:
Duplifinder is not compatible with Apple's new Photos app
Works with Mac OS X 10.8 or later and
iPhoto '08, '09 or '11
More info: Duplifinder Release Notes Duplifinder Manual
Duplifinder finds the duplicate photos and videos in your iPhoto Library.
It shows you each original with its duplicate(s):

Please Note: Duplifinder only finds exact duplicates, much like iPhoto does when you try to import duplicates. It will not match photos that are different size, cropped versions of other photos, "enhanced" or modified photos, etc.
"it’s become an essential tool to help clean out my Mac" - MacStories
"you can bet you’ll use the software often if you’re taking photos all day long." - MacStories
"having it available to you in a pinch will be huge." - Macgasm
"The interface could not be any simpler." - Mac360
"Duplifinder elimina fotos duplicadas no iPhoto" - MacMagazine