Holy Baptism is the basis of the whole Christian life, the gateway to life in the Spirit, and the door which gives access to the other sacraments. Through baptism we are freed from sin and reborn as sons of God; we become members of Christ, are incorporated into the Church and made sharers in her mission: "Baptism is the sacrament of regeneration through water in the word." CCC 1213
The Catholic Church teaches and professes one baptism for the remission of sin. To be valid, baptism must includes the words: "[N.], I baptize you in the name of the Father, and of the Son, and of the Holy Spirit," with three separate pourings of water (or immersions) at each name of the persons of the Holy Trinity.
Other Christian churches, such as the Orthodox and mainline Protestant churches use the same Trinitarian with water formula for baptism as the Catholic Church, therefore once verified, the Catholic Church accepts those baptisms as valid. However, we must be morally certain that such baptism occurred and will request documentation.
We do not "re-baptize" a person that is already validly baptized; in Catholic theology there simply is no such thing as "re-baptism." A valid baptism may only occur one time in a person's life because of the indellible spiritual mark or character that is imprinted on the person's soul. This spiritual mark or character cannot be removed, repeated, or forgotten; it consecrates the baptized person for Christian worship. Therefore, we will only baptize a person if it is determined that they were either (1) never baptized, or (2) were baptized in a formula not recognized by the Catholic Church.