I say, and with God's success: Washing the hands is obligatory once, and the order in washing them after the face is a Sunnah. So when we wash our hands at the beginning of ablution, the obligation of washing the hands is fulfilled. If one washes them again with the arms, it fulfills the Sunnah. If one does not wash them with the arms, then his ablution is valid, but he misses the reward of the Sunnah of washing them. And God knows best.