It would be nice to be offered a free or discounted ebook if you purchase the hard-copy format. But I suspect that most publishers (not Amazon) would never agree to this.
Amazon themselves have always been on-board with heavy discounting so I'm sure they'd be up for such a scheme if the publishers permitted it. It could certainly help to entice some more tree-book readers onto the ebook wagon.
With regard to buying ebooks where I've already bought the hard copy - paying twice for a product. Well it's the same as moving from video to DVD - different format comes along and you end up re-buying your favourites. The price of technology really.