So I got a free code for Double Dragon, which I already own.

I'm not clear on whether it's against the terms of service to give the code to someone else?

Yeah it's fine (well, GOG hasn't said anything so far, and I'm sure they would if it weren't).

Just one thing, in order to make it a bit more difficult for bots, don't just post the code here in the forum, but instead post the offer of the key, and anyone who's interested can then contact you so that you can give them the key in private.