Gnostic: bought 7 Pirtata, already own 4 of them.
The first 3 Pirtata give me games I never own, but the subsequent 4 give me games I already own.
So I think it will give you games you do not own first, then if you own everything it will choose randomly from those you already own.
If you buy till you received gift copies, you should stop. Unless you have some spare money to gift games.
That might not be a general rule ; the very first pinata I got was Machinarium Collectors Edition, which I already own here on gog. I don't think I'll try my luck with pinatas again, at least not until the containings of the pinatas change in 2 days...