TRy iPhoto Diet at Versiontracker. Seems to do a good job of getting rid of duplicates.
You're not looking in Finder are you? If so, those duplicates could be thumbnails. Whatever you do, stay out of the iPhoto library in finder. Do any and all work from within iPhoto app. Messing with Finder will cause untold grief -- lost photos, photos not usable within the app, corrupted photos etc. Forget that that file exists and use the app to do whatever you want with your photos.
Good luck!