Objection 1. It would seem that a man who is already under sentence of excommunication cannot be excommunicated any further. For the Apostle says (1 Corinthians 5:12): "What have I to do to judge them that are without?" Now those who are excommunicated are already outside the Church. Therefore the Church cannot exercise any further judgment on them, so as to excommunicate them again.
Objection 2. Further, excommunication is privation of divine things and of the communion of the faithful. But when a man has been deprived of a thing, he cannot be deprived of it again. Therefore one who is excommunicated cannot be excommunicated again
On the contrary, Excommunication is a punishment and a healing medicine. Now punishments and medicines are repeated when necessary. Therefore excommunication can be repeated.
I answer that, A man who is under sentence of one excommunication, can be excommunicated again, either by a repetition of the same excommunication, for his greater confusion, so that he may renounce sin, or for some other cause. And then there are as many principal excommunications, as there are causes for his being excommunicated.
Reply to Objection 1. The Apostle is speaking of heathens and of other unbelievers who have no (sacramental) character, whereby they are numbered among the people of God. But since the baptismal character whereby a man is numbered among God's people, is indelible, one who is baptized always belongs to the Church in some way, so that the Church is always competent to sit in judgment on him.
Reply to Objection 2. Although privation does not receive more or less in itself, yet it can, as regards its cause. In this way an excommunication can be repeated, and a man who has been excommunicated several times is further from the Church's prayers than one who has been excommunicated only once.
Source: Summa Theologica (New Advent)