Question: are duel deck cards legal in commander?
Answer: Any card in the duel deck that was printed in a Standard-legal set (Currently: Kaladesh, Aether Revolt, Amonkhet, Hour of Devastation, Ixalan, Rivals of Ixalan) and isn't on the Standard banlist will be legal in Standard. ... As long as the card isn't on the Commander banlist, it will be Commander-legal.