iPhoto can get sloppy with duplicate images, or even thumbnails. Especially for those who migrated from PC to Mac and used the auto import of iPhoto. Brattoo Propagando has you covered with their offering called Duplicate Annihilator and Thumbnail Annihilator.
Mac has a wonderful utility called Image Capture which will download your photos, movies, and music. Which will allow you to manage your media the old fashioned way.