These applications will identify and help remove duplicate photos from an iPhoto Library:
iPhoto Library Manager - $29.95
PhotoSweeper - $9.95 - This app can search by comparing the image's bitmaps or histograms thus finding duplicates with different file names and dates.
Duplicate Annihilator - $7.95 - only app able to detect duplicate thumbnail files or faces files when an iPhoto 8 or earlier library has been imported the library.
DeCloner - $19.95 - can find duplicates in iPhoto Libraries or in folders on the HD.
DupliFinder - $7 - shows which events the photos are in.
iPhoto AppleScript to Remove Duplicates - Free
PhotoDedupo - $4.99 (App Store) - this app has a "similar" search feature which is like PhotoSweeper's bitmap comparison. It found all duplicates
Duplicate Cleaner for iPhoto - free - was able to recognize the duplicated HDR and normal files from an iPhone shooting in HDR
Some users have reported that PhotoSweeper did the best in finding all of the dups in their library: iphoto has duplicated many photos, how...: Apple Support Communities.
If you have an iPhone and have it set to keep the normal photo when shooting HDR photos the two image files that are created will be duplicates in a manner of speaking (same image) but there are only twp apps that detected the iPhone HDR and normal photos as being duplicates: PhotoSweeper and Duplicate Cleaner for iPhoto. None of the other apps detected those two files as being duplicates as they look for file name as well as other attributes and the two files from the iPhone have different file names.
iPLM, however, is the best all around iPhoto utility as it can do so much more than just find duplicates. IMO it's a must have tool if using iPhoto. Purchasing it at this time qualifies you for a license for PowerTools, the iPLM equivalent for the new Photos app.
