We continue with Vatican II directives on the theme of baptism: how to receive formally (ie, through the clergy) those who have already been baptized:
In place of the rite called the “Order of supplying what was omitted in the baptism of an infant,” a new rite is to be drawn up. This should manifest more fittingly and clearly that the infant, baptized by the short rite, has already been received into the Church.
And a new rite is to be drawn up for converts who have already been validly baptized; it should indicate that they are now admitted to communion with the Church.
And this latter provision accounts for half to three-fourths of RCIA work in my experience.