Duplicate Annihilator is a $7.95 shareware application that compares the photos of your iPhoto library to locate duplicate copies of the same picture. iPhoto offers some basic duplicate detection when importing photos, but that feature compares images on very limited information. It also doesn’t enable you to search your entire iPhoto library after the fact. So if you have larger numbers of pictures that you’ve added to iPhoto from various sources (such as files from other computers, backups, or websites), you can end up with multiple copies of one or more images pretty easily.
Duplicate Annihilator can compare images using a variety of techniques, including file checksum data, creation date, file name, file size, and the height and width of images. You can choose one or more of these criteria to attempt to locate images. This provides a solid chance of locating images that are actually duplicates. Duplicate Annihilator can also compare images that you suspect of being duplicates of each other. When searching for duplicate images, you can elect to have Duplicate Annihilator automatically move all the duplicates it finds to the Trash or set the comments for a photo to indicate that is a duplicate. (You can then search in your iPhoto library for the duplicates and decide to keep or remove them.) Its capability to clean up an iPhoto library (particularly a very large one) makse Duplicate Annihilator a great tool for helping to organize your photos and reclaiming the disk space used by duplicate photos.