Drakon135790: Yeah, I checked and I should have received the code in a gog message, or not been able to use the code from Amazon at all as you mentioned yourself, but none of that happened. The code from Amazon was treated as if I had used the code for a game new to my account and so just used it even though I already had the game on my account. I apologise for this as I assume you wanted to redeem it.
Oh, don't worry, I've been expecting it to fail; I suggest to report the bug to GOG staff, perhaps they "give you back" the code, so you can give it away again – or even better: they might fix that bug. They won't, if nobody complains…