I've won a giveaway twice, how do I request a new winner?

Let the creator know by leaving a message to re-roll your win on the GA, or add them on steam to explain they need to re-roll.

Just let the GA creator know.
Mark one as received and notify the other

Leave a comment in one of the GAs saying "I won this game twice. Please allow the giveaway creator to reroll." or such.
That's the first place support will look when checking if the GA creator has a valid reason to reroll.
Any other method tends to just cause hassle and delays, so unless the creator has you blacklisted, take that approach.

Note the giveaway creator has to contact support for the actual reroll request- your responsibility is just leaving them that excuse to do so.

Thx for the quick responses.