Objection 1. It seems that Baptism may be reiterated. For Baptism was instituted, seemingly, in order to wash away sins. But sins are reiterated. Therefore much more should Baptism be reiterated: because Christ's mercy surpasses man's guilt.
Objection 2. Further, John the Baptist received special commendation from Christ, Who said of him (Matthew 11:11): "There hath not risen among them that are born of women, a greater than John the Baptist." But those whom John had baptized were baptized again, according to Acts 19:1-7, where it is stated that Paul rebaptized those who had received the Baptism of John. Much more, therefore, should those be rebaptized, who have been baptized by heretics or sinners.
Objection 3. Further, it was decreed in the Council of Nicaea (Can. xix) that if "any of the Paulianists or Cataphrygians should be converted to the Catholic Church, they were to be baptized": and this seemingly should be said in regard to other heretics. Therefore those whom the heretics have baptized, should be baptized again.
Objection 4. Further, Baptism is necessary for salvation. But sometimes there is a doubt about the baptism of those who really have been baptized. Therefore it seems that they should be baptized again.
Objection 5. Further, the Eucharist is a more perfect sacrament than Baptism, as stated above (Question 65, Article 3). But the sacrament of the Eucharist is reiterated. Much more reason, therefore, is there for Baptism to be reiterated.
On the contrary, It is written, (Ephesians 4:5): "One faith, one Baptism."
I answer that, Baptism cannot be reiterated.
First, because Baptism is a spiritual regeneration; inasmuch as a man dies to the old life, and begins to lead the new life. Whence it is written (John 3:5): "Unless a man be born again of water and the Holy Ghost, He cannot see [Vulgate: 'enter into'] the kingdom of God." Now one man can be begotten but once. Wherefore Baptism cannot be reiterated, just as neither can carnal generation. Hence Augustine says on John 3:4: "'Can he enter a second time into his mother's womb and be born again': So thou," says he, "must understand the birth of the Spirit, as Nicodemus understood the birth of the flesh.... As there is no return to the womb, so neither is there to Baptism."
Secondly, because "we are baptized in Christ's death," by which we die unto sin and rise again unto "newness of life" (cf. Romans 6:3-4). Now "Christ died" but "once" (Romans 6:10). Wherefore neither should Baptism be reiterated. For this reason (Hebrews 6:6) is it said against some who wished to be baptized again: "Crucifying again to themselves the Son of God"; on which the gloss observes: "Christ's one death hallowed the one Baptism."
Thirdly, because Baptism imprints a character, which is indelible, and is conferred with a certain consecration. Wherefore, just as other consecrations are not reiterated in the Church, so neither is Baptism. This is the view expressed by Augustine, who says (Contra Epist. Parmen. ii) that "the military character is not renewed": and that "the sacrament of Christ is not less enduring than this bodily mark, since we see that not even apostates are deprived of Baptism, since when they repent and return they are not baptized anew."
Fourthly, because Baptism is conferred principally as a remedy against original sin. Wherefore, just as original sin is not renewed, so neither is Baptism reiterated, for as it is written (Romans 5:18), "as by the offense of one, unto all men to condemnation, so also by the justice of one, unto all men to justification of life."
Reply to Objection 1. Baptism derives its efficacy from Christ's Passion, as stated above (2, ad 1). Wherefore, just as subsequent sins do not cancel the virtue of Christ's Passion, so neither do they cancel Baptism, so as to call for its repetition. on the other hand the sin which hindered the effect of Baptism is blotted out on being submitted to Penance.
Reply to Objection 2. As Augustine says on John 1:33: "'And I knew Him not': Behold; after John had baptized, Baptism was administered; after a murderer has baptized, it is not administered: because John gave his own Baptism; the murderer, Christ's; for that sacrament is so sacred, that not even a murderer's administration contaminates it."
Reply to Objection 3. The Paulianists and Cataphrygians used not to baptize in the name of the Trinity. Wherefore Gregory, writing to the Bishop Quiricus, says: "Those heretics who are not baptized in the name of the Trinity, such as the Bonosians and Cataphrygians" (who were of the same mind as the Paulianists), "since the former believe not that Christ is God" (holding Him to be a mere man), "while the latter," i.e. the Cataphrygians, "are so perverse as to deem a mere man," viz. Montanus, "to be the Holy Ghost: all these are baptized when they come to holy Church, for the baptism which they received while in that state of error was no Baptism at all, not being conferred in the name of the Trinity." On the other hand, as set down in De Eccles. Dogm. xxii: "Those heretics who have been baptized in the confession of the name of the Trinity are to be received as already baptized when they come to the Catholic Faith."
Reply to Objection 4. According to the Decretal of Alexander III: "Those about whose Baptism there is a doubt are to be baptized with these words prefixed to the form: 'If thou art baptized, I do not rebaptize thee; but if thou art not baptized, I baptize thee,' etc.: for that does not appear to be repeated, which is not known to have been done."
Reply to Objection 5. Both sacraments, viz. Baptism and the Eucharist, are a representation of our Lord's death and Passion, but not in the same way. For Baptism is a commemoration of Christ's death in so far as man dies with Christ, that he may be born again into a new life. But the Eucharist is a commemoration of Christ's death, in so far as the suffering Christ Himself is offered to us as the Paschal banquet, according to 1 Corinthians 5:7-8: "Christ our pasch is sacrificed; therefore let us feast." And forasmuch as man is born once, whereas he eats many times, so is Baptism given once, but the Eucharist frequently.
Source: Summa Theologica (New Advent)