Thanks for the replies! I guess the best thing would be to always buy them as gift codes first then? Or do you think that makes it more likely that you'll get games you own already, because GOG thinks it doesn't matter for gifts?
coffeecup: If you buy multiple pinatas with one purchase and there is it at least one dupe, they all get stuck into one single gift code (including the game(s) you do not own), so beware.
Currently there is a support ticket since Friday open for me, so I hope my reply to their answer is being processed soon (tm) ... well really, I don't think so that I'm not alone :)
You can redeem each code individually though and the games will be added to your account only after you've confirmed the ones you want to redeem. Still a little inconvenient if you want to gift to (or trade codes with) more than one person, because you'll have to trust them not to redeem the games that weren't meant for them. But just in case this is what your support request was about, it should be no problem for you to redeem the games you don't own, and the other ones will remain on the code.
coffeecup: I don't understand the people who complain that they got games which they already own on Steam or somewhere else. GOG has no relations to these other companies and no, you do not get steam codes, why even?
It's no reason for complaining after you've bought them, but it is a valid reason to stay away from the pinata sale, especially since quite a few of the games have been in bundles and are therefor likely to be owned already by indie game fans who don't shop at GOG.com exclusively.