When one vivi ornitier is in the graveyard and you exile it with Agatha's soul cauldron and target another Vivi in play giving it the same ability twice. You should be able to activate each ability once per each of your turns being they are two different abilities even if they are identical. I have had multiple times where I try to do so but the game won't allow me to activate the second ability and sometimes even ending my turn automatically if it's my second main phase.