Nowadays iPhoto protects you from importing duplicates but if you are like me, you have somehow managed to do so anyway.

Duplicate Annihilator takes on the time consuming task comparing the images in your iPhoto library using effective algorithms to make sure that no duplicates escapes.

When found the duplicate will either be marked with a description of your choice to make it searchable or simply moved to iPhotos trashcan. With Duplicate Annihilator you can remove duplicates quickly and easily.