VileOne: Interesting. That is one stupid bug. I hope it will be fixed soon. Thanks for checking! :)
Titanium: If that's the case, I don't think it's a bug. It's probably a standard "promos or discounts don't stack". The bigger question is why don't you get the appropriate feedback, like informing you straight away that you can't do that and why.
Actually, that sounds more like a bug. The promo code or gift code entitles the user to one copy of the game, assuming that the code hasn't been used already. I'm not sure I see why it would matter whether the game was on sale or not. GoG has the money already and you have the game after redemption.
The only place where I could see it making a difference is when buying as GoG probably doesn't want to be perpetually competing against previous sales.
