Just played a guy who had 2 copies of multiple legendary cards in his deck, he had two master of possibilities as well as two approach of the second sun. As per the deck building rules he should not have been able to do either. He did not draft at all during the game nor add random spells or creatures to his hand. He was playing as Jace.
Master of Possibilities and Second Sun's Dawn are Mythic rarity cards which do not have the Legendary keyword. You can play up to 2 copies of non-Legendary cards in your deck (including Mythics), but you can only play 1 copy of a Legendary card.