So, you are snagging stuff from Photo Stream, and that is duplicating?
That makes sense. You can't modify a photo in Photo Stream, and you can't keep using it as a base reference for non-destructive changes in a photo, not when it might go away in how ever many days or photos. That means that iPhoto has to create a local copy. You'll probably get a new copy every time you open a Photo Stream file in iPhoto.
It probably does not affect the pictures taken with the iPad because you do have a local file in the camera roll. Try deleting the local file, and see if Photo Stream files on the iPad start duplicating.
To avoid it, try saving a copy to the camera roll first, then using that in iPhoto.