Objection 1. It would seem that this sacrament ought not to be repeated. For the anointing of a man is of greater import than the anointing of a stone. But the anointing of an altar is not repeated, unless the altar be shattered. Neither, therefore, should Extreme Unction, whereby a man is anointed, be repeated.
Objection 2. Further, nothing comes after what is extreme. But this unction is called extreme. Therefore it should not be repeated.
On the contrary, This sacrament is a spiritual healing applied under the form of a bodily cure. But a bodily cure is repeated. Therefore this sacrament also can be repeated.
I answer that, No sacramental or sacrament, having an effect that lasts for ever, can be repeated, because this would imply that the sacrament had failed to produce that effect; and this would be derogatory to the sacrament. On the other hand a sacrament whose effect does not last for ever, can be repeated without disparaging that sacrament, in order that the lost effect may be recovered. And since health of body and soul, which is the effect of this sacrament, can be lost after it has been effected, it follows that this sacrament can, without disparagement thereto, be repeated.
Reply to Objection 1. The stone is anointed in order that the altar may be consecrated, and the stone remains consecrated, as long as the altar remains, hence it cannot be anointed again. But a man is not consecrated by being anointed, since it does not imprint a character on him. Hence there is no comparison.
Reply to Objection 2. What men think to be extreme is not always extreme in reality. It is thus that this sacrament is called Extreme Unction, because it ought not to be given save to those whose death men think to be nigh.
Source: Summa Theologica (New Advent)