can a person re-participate in winning a gift

If you own it, you are not eligible for a win.

Yes, you can win the same title twice if you didn't activate your previous win.
You can still activate and join the giveaways of the same title if SteamGifts didn't sync with your account.
However, the winner will be punished if this happens.

You can't enter a GA for a game which you already won (it's not possible at all, it's blocked - it used to be technically possible, but this was removed a long time ago, maybe 2 years or so)

You "can" technically enter a GA for a game which you already own (if it's hidden by the Steam API for some reason), but it's not allowed and can get you in trouble (if you win a game you already own by accident, just don't touch it and instead ask the GA creator for a reroll)

If the peron has previously won the game, they should not be able to enter to win that particular game again. As the poster above said, you should not enter for games already owned.
I check all winners with sgtools for unactivated wins or multiple wins before handing the winner the key.

I am not sure what you are asking but in case this is what you mean:

As soon as you win a game here on this website you can no longer join in on any giveaways of that same game because you won it already, unless it gets marked as Not Received.

I have friends who have this game and they don’t have it!

As the lucky winner of the first Cyberpunk 2077 giveaway in Steamgifts I can tell you that no, you can't enter other Cyberpunk 2077 giveaways.

let the moderators write about this shit https://www.steamgifts.com/user/Exilarch

let the moderators write about this shit https://www.steamgifts.com/user/Exilarch

Could it be because it is not released yet it doesn't show in steam profile "Games" despite owning it ?

Never owned a pre-order before so can't really tell.

Calling out someone is strictly forbidden by the rules, you should delete/edit all your posts pointing to some user profile, or you'll be punished by the mods.
If you spot a user breaking the rules, report him through a support ticket, but never call out, whatever happens.