If someone uses a 12 element rainbow, they lose, because they have no decks to play after. Why would that be a problem?
A couple of weeks ago, you had to have opposing elements, and use a different deck every duel. If you had only one deck, you lost your next duel.
In this case, you cannot reuse the same element twice in a round. If you can't build decks to do that, you will lose, again. That doesn't seem to be a problem, just a part of the rules.
As for the mark, as I struggled with in my first tourney, it seems that marks are not part of the deck, and, as such, don't follow the rules of not repeating, (just like in the duality tournament, you could use earth/air with time mark), so I would assume you could use, say, earth with a time mark, then time with a time mark here, since the mark isn't part of the deck, and, so, not an element that you are using.
However, confirmation would be nice, since, as I said, I struggled with this in my first tourney.