/summa/XP/XPII/XP_Q22:
We must now consider those who can excommunicate or be excommunicated. Under this head there are six points of inquiry:
(1) Whether every priest can excommunicate?
(2) Whether one who is not a priest can excommunicate?
(3) Whether one who is excommunicated or suspended, can excommunicate?
(4) Whether anyone can excommunicate himself, or an equal, or a superior?
(5) Whether a multitude can be excommunicated?
(6) Whether one who is already excommunicated can be excommunicated again?