Question about winning key I already owned

Only the creator can ask for a reroll so you really can't do anything from your situation.
And what to do... I'm really not sure ;
If you mark it as received it will change nothing because as you said, an api-removed game, that's why you could even enter for it.
or you can mark it as not received and let the creator sort it out if he even wants to do so.
From my point the first option is not exactly morally correct but they can't be bothered and there won't be any consequences, while the second one is more right but makes extra work for someone who don't care. I think it's up to you which option do you choose. (Or someone appears and tells me if I'm wrong at some points, so you'll get more info about the situation :D

There's nothing about "view button" in FAQ, so i think u can ask about reroll and mark as Not recieved after 7 days

So in TLDR: simplest but not nicest way: thank the creator, mark it as received and don't give a f**k about it. Any other options and tries to keep the rules end up more work and just as untraceable.

You could screenshot the "game is already in library" message as a proof it's a working key, maybe?

Nevermind then.

You can mark it not received after 7 days if the creator refuses to re-roll. The copy you activated yourself will show in your licenses and activations, along with the date activated if you need proof for support.

There are several games (removed from Steam) that still show up on SG as unowned because SG has no way of checking.

Funny thing: I notice that on this GA I specifically added a comment saying "careful, it's not detected, check in your Steam client" 🙃

Got scammed once by a winner who pretended he already had the game whereas he actually activated my key, so I can understand the GA creator's stance. Since that time, in my GAs I always write in the description "please double check that you don't already own the game before revealing the key".

Is it safe to just mark the key as "received", and not use it?

Since the game is indeed in your library, yes it is, as long as you haven't won a previous GA for it before.