keeveek: At least on GOG, the price is reduced if you have a game here. And why GOG should allow you to untick games you have elsewhere? Does any other vendor do that? Don't think so.
tinyE: That is actually something similar to what retail stores do over in the States (we call it price matching and I don't know if they do it in Europe) and it works but I think it might too easily be abused with the internet. If you go to WalMart with proof that you can get something cheaper at ShopCo then they match the price.
Keevek was talking about different thing, ie. that WalMart has a deal that you can buy two jeans for a 50% discount, single jeans will get only 20% discount. You go to their store and show a receipt that you bought same jeans (one pair) from them already two weeks ago, so they agree to give you full 50% discount for one pair of jeans, instead of 20%.
As for price matching, some shops had it here in Finland too (e.g. I remember one PC games shop saying they'll pricematch any other store; and when I did that, they simply called the other shop as a fake customer to check whether the price is what I claim).
Anyway, as far as I understand, the whole pricematching practice was later deemed illegal here, and stores are not allowed to do that here anymore. I think it even is about customer protection, ie. the stores will have to offer a cheaper (cheapest?) price to everyone, not only those who are active enough to shop around and check prices from various stores.
I guess the reason is that when some store says they'll price-match any other store, most customers incorrectly take it to mean that whenever some customer tells the store that item A is cheaper somewhere else, the store will lower the price for everyone from then on. That is not the case, they only lower it to that one active customer, and keep the higher price for everyone else. So, passive customers pay extra at the same store, with the wrong expectation that they are getting the same price-matching as everyone else.