If you go to Settings > General > Usage, it will show under Storage how much memory is used by Photos & Camera, and when you tap that, it will show you what portion is from your Photo Stream. Now, I had a shared photo stream with about 20 photos, and I actually deleted it, and when I checked I saw that Photo Stream storage did not go down. So I suspect if the same photo is in multiple photo streams, it does not use extra storage on your device. What it did imply, however, is that each photo in my Photo Stream is using about 1MB, and I know full sized photos from the iPhone are usually closer to 3MB. Apple says that photos downloaded to the Photo Stream on mobile devices are scaled down to use less space.
So this all indicates that the Photo Stream includes a duplicate copy of each photo, but a single photo in multiple photo streams does not increase storage.
What I personally recommend is importing photos and videos to iPhoto or Picasa and using that software to delete them from the iPhone every once in a while. This clears the camera roll only. That keeps storage down while still letting you view recent photos in your Photo Stream.