Interesting question.

The proper procedure is you activate one of them, and request a reroll for the other by informing the creator of it in a comment in his giveaway. If you mark both as received, you'll break a rule.

Then you tell one of the gifters to ask for a re-roll. As you have to add all your wins to your personal library, you would break that rule, if you kept the second win.

You should ask to request a new winner. Since you can only win 1 copy from 1 game

Request a reroll, preferably in the giveaway that had more entries.

not that interesting

As stated above, you're required to activate all wins to your own account, so having multiple wins of the same game just means you'll need to get one rerolled [or end up having a non-activation on your record].

But further:
Make sure to comment on the giveaway you want rerolled, stating clearly that you wish such. This will make it a lot easier for the GA creator to get their reroll from staff, versus if you approach it in another manner.
If the GA creator doesn't pursue the reroll with staff, you can't insist on it, as only the GA creator can initiate that process. In that circumstance, the correct response is to simply leave the giveaway without a 'Received' or 'Not Received' selection- just leave it unmarked, which will indicate the responsibility for the giveaway is still in the hands of the GA creator [and, if it passes the one week mark without you having marked feedback on the giveaway, they have grounds to reroll anyway (though it's clearly rude to take that approach without saying anything to the GA creator about the matter)].

it only gets interesting when people are suspended for multiple wins :3