Catechism of the Catholic Church §1317 — excerpt
Confirmation, like Baptism, imprints a spiritual mark or indelible character on the Christian's soul; for this reason one can receive this sacrament only once in one's life.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.