What if I won 2 same game

You can't. When you win a game, you're automatically taken out of all other giveaways you entered for the same game.

Winning a game will automatically remove your entries for other copies of the game - unless you manually enter them again, you can not win the same game twice.
If it would happen, you can always leave a comment on the giveaways to ask the creator to reroll the giveaway - just don't reveal it's key.

It can happen if two giveaways for the same game end at exactly the same time. Just wait and see who sends the game first and once you've verified that it works, ask the other guy to reroll by leaving a message at their giveaway 🤣
Make sure you mark the first one as received after you activate it, but do not mark the second, or you will get in trouble.

happened to me once but the 2nd GA I won was luckily closed on a later date so I was able to ask owner to re-roll.

What happens if somebody else gets rerolled and you happen to get it that time but have since added the game to your account?

Have had the same trouble and since then I am out of any SGTools-GAs because the owner didn't want to reroll -.-

It has happened to me. I accepted the first one. For the second one, I wrote "I recently won this in another giveaway - please reroll" in the comments. This worked out fine. It's straightforward for the giveaway creator to ask for the winner to be rerolled.

This should no longer be possible. It used to be in the past, but now you can't enter giveaways of games you won till it's marked not received, which requires AT LEAST a week, or the game got re-rolled.