I used a program called Twins... I pulled all the files from the iPhoto directory into the program, it detected the duplicates and I selected automatically delete.
I think they all have free trials where you can scan for free, so you don't have to pay upfront unless they find what you're looking for.
I'd recommend using an external hard drive to download to and run these apps from. Any more read/writes to your current drive can overwrite some of the already-deleted data.