You don't have to participate if you don't want to. The charity police won't come to the door if you bah-humbug this. It's purely voluntary, and most people do it for the fun of it.
I haven't heard any procedure concerning being challenged a second (or more) time. Me, I'd just say, "Okay, done," and link to the one I already completed.
As for not accepting a triple dog dare, that's serious. You lose all your playground cred. Better to get your tongue stuck to a pole.