When you purchase a new physical book, your receipt should have a download code so you can read it on your Kindle or phone as well.

I love owning my books, and I read them at home, but if I'm going on a trip or it's a particularly big (heavy) volume I'll read it on my Kindle. I don't see why I should have to buy it twice just because I like both collecting and convenience. eBooks can still be offered as the cheaper option, and you could maybe limit free eBooks to people who get it in hardcover or something, if they're worried about profit.

Edit: Yes of course this means you can just sell the book later on and keep the digital version or vice versa but the used book market already exists anyway, and nobody will ever be able to 'scalp' a book.

Also, If this idea would ever really become a thing, I imagine you'd either have to pay extra (2-5 dollars) to get the download code or the book would simply cost more up front. I didn't think that publishers would give me a second 'copy' for free, just less than what it costs to do now.