Why Chesterton's baptism could be called "unfortunate"

Lane's explanation:

When I wrote "unfortunate, I think", I meant that Chesterton should not have been baptized when he joined the Catholic Church, because he had already been baptized as an infant in the Church of England. According to Catholic understanding, too, baptism is what incorporates one into Christ's Church. Any baptism according to the rites used in a mainstream Protestant church (like CoE) is a valid baptism, therefore, according to Catholic understanding.

It had, however, become a custom, at least in England and America, to indiscriminately baptize adult Protestants who were joining the Catholic Church on the grounds (largely unlikely, it seems to me) that the prior baptism might not have been valid; and, better safe than sorry, I guess.

This practice has been repudiated since Vatican II, most pointedly in the Instruction of the Rite of Christian Initiation of Adults promulgated by Rome in 1972, and baptism is therefore not usually administered to converts from a Protestant church these days unless it is actually established that the person had not been baptized at all or that it had not been a real Christian baptism (which would be the case if, say, a liquid other than water was used, or if only Jesus' name was used rather than an invocation of the Holy Trinity). I myself was baptized in a Methodist church when I was three months old, so I was not baptized when I joined the Catholic Church at seventeen.

Sometimes, in the "old" days, the baptism of a convert would have been done conditionally, with the presumption that the person had already been baptized validly but allowing for the possibility that he had not been. The words used would take a form like "If you are not baptized, I baptize you in the name of the Father, and of the Son, and of the Holy Spirit". This is still done if there is reasonable doubt that the person had been baptized validly, since baptism is necessary but it is considered sacrilegious to "re-baptize" somebody who is already baptized.