win the same game multiple times

you're allowed to enter any giveaway you want, essentially you cannot win the same game multiple times can be broken down to this:
when you win a giveaway the win is not absolutely yours, if you tell the creator you won it at another giveaway he will reroll and you will maintain 1 win for that game.
you can also remove all entries for the game that you've been trying to win, in the search function you can type dark souls 3, and enter 20 different giveaways, assuming you won one of them, you can remove your entry from the others and get your points back.
i advice reading this aswell https://www.steamgifts.com/about/faq

If you win a game you already own (due to two giveaways for the same game that ended at the same time, or due to you forgetting you had an entry still up for a game you won earlier on), just leave a comment in the giveaway in question saying that you would like the creator to reroll. So long as you've made that comment, staff will permit the creator to reroll, and you won't get into any trouble.

Since you're required to activate all wins onto your own account, if you win multiple copies of the same game, clearly that presents an issue, as you can't possibly activate the same game multiple times. As such, you get a multi-win violation on SG at that point [which is simply a subset of non-activation as far as SG rules are concerned, and is suspension-meriting as a violation of that rule].

As far as simply entering for the same game (that you don't yet own) goes, that's not only permissible, it's expected.

Also, if the giveaway creator doesn't reroll in a reasonable length of time (as happened to me once, despite repeated attempts to contact them by various channels), you should probably ultimately just mark the redundant game as not received. It's a shame to have to do so, but it'll keep your record clean.