Policy on gifting?

You got it right, if the winner already owns the game you should ask for a re-roll.
Note that you cannot enter GAs for games you already own, so this is a rare case where someone got the game after entering the GA. (or someone who has a private profile and never syncs)

you even have to ask? ofc reroll! you don't even have to contact him, message support

If the game you win is not activated on your account, it will result in a ban.

Re-rolling is what you generally do, yes. I had to do this a few times when I was part of one of the big Fortix giveaways in the past, where some of my winners won multiple copies.

Usually the sync requirement avoids such situations, but yeah.

To re-roll, go to your giveaway, click on the text in the sent column for your giveaway, and click on Request New Winner. You write in a reason and the request is sent to support for approval. They seem to respond pretty quickly, which is nice.

Ah, got it. Thanks a lot guys, been a huge help.