Help: I Won The Same Game Twice in One Day (Solved)

Unmark it. Contact GA owner. Tell them to reroll.

You can unmark it. Add him on steam or comment on his giveaway.

When the reroll happens, it won't even appear in your won items anymore. It'll just be gone with no sign :)

This happened to me, same game, from the same person, 3 minutes apart lol.

In general, you'll want to comment directly on the giveaway itself with a comment in line of "I agree to cancel" or "I agree to a reroll".

This is the first place the giveaway creator and SG staff generally look for that kind of thing, and SG staff won't reroll without that sort of confirmation.
If you post the request anywhere else, the giveaway creator can still get a reroll, but must take a screenshot and link that to staff, first.

Thus, posting on the giveaway directly makes things a lot easier on all involved. :)

There's no fault in leaving a giveaway unmarked either way, by the way-
If you do that, the giveaway creator will automatically need to contact you after a week anyway- if you keep ignoring them, they can get their reroll regardless. (Obviously, this is a more rude option than trying to sort it out faster).
If you mark it either way, however, you're going to potentially get one or the other of you in trouble, so try not to mark unless you explicitly did not receive a game, the game you got was incorrect/not-working, or you did receive the exact game listed. Unmarked is basically a 'neutral' option between Received and Not Received :)

Also, as noted by Navaro, you can input the name of the game into the search box in the top left of the Giveaways Entered tab, and make sure you don't miss any. :)