To take an extreme example, if a person is on a plane that's about to crash, and she wishes to perform a baptism that is valid according to the Catholic Church (and therefore valid according to the many other churches that have the same practices of baptism), should she open the nearest water bottle and:
Say "I baptize you all in the name of the Father, and of the Son, and of the Holy Spirit" while splashing the water towards the heads of the intended recipients of baptism, thus baptizing at least those who had not already been baptized? Or should she:
Say "I baptize you in the name of the Father, and of the Son, and of the Holy Spirit" while pouring the water onto the head of the nearest person, and then proceed to the next person, and so on until death,
because the first option would not be a valid baptism? (Assume that this person is a child that cannot hope to land the plane, that she believes in the same God as Catholics, that she intends to bring the recipients of baptism into the fullness of Christ's Church, that air pressure is lost leaving no more-competent passenger conscious, and that the would-be recipients are a large number of lapsed family members, some of which have been putting baptism off for so long that there is reason to believe that their desire for baptism is at least questionable...)
Put a bit more plainly:
If a Catholic wishes to administer a valid baptism to many people in danger of death, must she baptize each person separately, or can she baptize all of them in one act, and what would be the correct form for doing so?