Well here's the thing: the key is from a fresh bundle and I revealed it immediately before creating this GA. I checked all the other GAs I created at the time to see if I didn't mistakenly put the same key twice, but no. After all my other GAs from that bundle are ended and the keys received, I will contact bundle stars to see what they have to say about this supposedly duplicate key. I don't have any means to check it as I already own the game so it won't show the message. Does it show to you as duplicate, or invalid, or something else?

Ok I asked a friend to try it and it does say duplicate. I'll contact bundlestars and see what they say.

bundlestars sent a replacement key, I added it for you, please check it and tell me if it works.