What should i do if duplicate steam key and blacklisted? [SOLVED]

Yes or according to SG rule you will get banned for not activating the game. Happened to me 9 years ago marked a game received but the key was duplicate and i got banned for 2 days i think. I was new.

Yes. When you are able mark it not received. Do not mark it received unless it is exactly the game in giveaway.

you can comment on his steam profile, if he will want care. Otherwise there is no other way than marking as game not recieved

Look at his Won GAs. If he left a 'Thank you' note in one of them, you can reply to it.

Leaving a comment on someone's Steam profile referencing SteamGifts is risky because it can lead to a Community ban.

If you are blacklisted by giveaway creator, you can still view the giveaway page in incognito mode because you are not logged into your account. It will not work with whitelist or group giveaways, but it works with public or invite only giveaways. This is most useful with community trains, as there are many giveaways with different creators.