Just make sure you are deleting exact duplicates. If you are looking in the iPhoto library in finder, you are seeing "originals", "modified" and "data" or "thumbs". DO NOT DELETE ANY OF THESE! Stay out of the library in finder and work only in iPhoto app (if you want to use iPhoto at all). If you disturb any of the photos in the library, iPhoto will not be able to work for you.
Now, if you actually have duplicate photos that you see when you are working in the app itself, then Duplicate Annihilator may well be what you want (I have not used it, so can't attest one way or the other).
Just stay out of the finder!