No. 1280 and 1281, page 325 of the Catechism of the Catholic Church: The Celebration of the Christian Mystery; Libreria Editrice Vaticana: Urbi Et Orbi Communications:
Baptism imprints on the soul an indelible spiritual sign, the character, which consecrates the baptized person for Christian worship. Because of the character Baptism cannot be repeated (cf. DS 1609 and DS 1624).
Those who die for the faith, those who are catechumens, and all those who, without knowing of the Church but acting under the inspiration of grace, seek God sincerely and strive to fulfill His will, are saved even if they have not been baptized (cf Lumen Gentium 16).
Q. Should one who, in case of necessity, has been baptized with private baptism, be afterwards brought to the Church to have the ceremonies of solemn baptism completed?
A. One who, in case of necessity, has been baptized with private baptism should afterwards be brought to the Church to have the ceremonies of solemn baptism completed, because these ceremonies are commanded by the Church and bring down blessings upon us.
[NOTE: There seems to be a contradiction between paragraph 1280 of the new Catechism of the Catholic Church and paragraph 648 above; however, the new Catechism is talking about "formal" baptism not being performed again. Emergency baptisms (Conditional Baptism) that often take place at the birth of a child in danger constitute baptism, but the child is later received formally into the Church through solemn baptism which is absolute as opposed to "conditional."]
Q. What is Baptism of desire?
A. Baptism of desire is an ardent wish to receive Baptism, and to do all that God has ordained for our salvation.
Q. What is Baptism of blood?
A. Baptism of blood is the shedding of one's blood for the faith of Christ.
Q. What is the baptism of blood most commonly called?
A. The baptism of blood is most commonly called martyrdom, and those who receive it are called martyrs. It is the death one patiently suffers from the enemies of our religion, rather than give up Catholic faith or virtue. We must not seek martyrdom, though we must endure it when it comes.
Q. How do we know that the baptism of desire or of blood will save us when it is impossible to receive the baptism of water?
A. We know that baptism of desire or of blood will save us when it is impossible to receive the baptism of water, from Holy Scripture, which teaches that love of God and perfect contrition can secure the remission of sins; and also that Our Lord promises salvation to those who lay down their life for His sake or for His teaching.