Automatically withdrawn from future giveaways upon winning?

No.

If you don't update your Steam account, you can still enter giveaways of games you own.

But if you are caught, you are a dead man.

I don't think it does this.

You need to withdraw yourself or have the guy reroll the gift if you forget if I am not mistaken.

Super unlikely but no I don't think you're automatically withdrawn. I really wouldn't worry bout it though.

It will display that you already own the game and you won't be able to submit an entry. This does not apply if the game is put in a bundle or a combination pack, including if it's a combination of base game+DLC for that game. You can also go under account and sync your account to display the newly activated game, and then also under account click filter games. Giveaways with the game will not be displayed then.

If you've won and you also have entries for that game in a giveaway that ends later it will still count you as entered and deduct points when the giveaway is over.

You're not automatically withdrawn. There have been people who have won the same game twice from separate giveaways.

It's also not a good idea because some giveaways are fake. So if someone wins a fake giveaway, then gets kicked from a legitimate one for the same game, they will be angry.

Automatically withdrawing people from giveaways when that game appears in their Library is a better idea though.

all u should do is goto ur entries and cancel any giveaways u have entered of the game u just won, its not hard!