How do you get Acerus if already got one but want another and the azure flute is not in your invetory?

To get another Acerus, you must be in possession of the Azure Flute. After you obtain the flute, go to Route 114 and encounter a wild Sawsbuck. Play the Azure Flute while in battle, and Acerus will descend from the sky to battle you. After defeating Acerus, you can capture it and add it to your team.