It is the teaching of the Catholic Church that certain Sacraments leave a mark (character) upon the soul which can never be erased ... or duplicated. An apostate may renounce their baptism with all the formality they can devise .... but they are still baptised and, if they repent, will be absolved but never rebaptised. A disgraced priest may be laicised and forbidden even to dress as a priest, but he is still a priest and, in extreme circumstances, may absolve the dying. (Confirmation is the third such Sacrament.)
But what if there is some doubt about the validity of a Sacrament? That doubt needs to be removed; but simply to repeat the Sacrament would be sacrilege if the original administration of that Sacrament was, after all, valid. So the Sacrament is administered sub conditione; Si non es baptizatus, ego te baptizo etc..
I believe there are two areas where Conditional Administration ought to be part of the Church's normal practice. The first regards the baptism of converts. In a less ecumenical age, converts were always conditionally baptised in England in case their baptisms had not been adequately carried out in another ecclesial body. But nowadays, since there is no doubt that Anglican baptism, according to the rites authorised in the Church of England, is certainly valid, current Catholic praxis rightly accepts it.
But these assumptions are no longer safe. We hear of fashionable Anglican churches with fashionable, indeed episcopable, clergy where, contrary to the rules of the Church of England, baptism is invalidly done in the name of Creator, Redeemer, Sanctifier. It is probable that such aberrations will become more, not less, common (indeed, an Australian Catholic parish proved to have been doing exactly the same). The baptismal register of such churches will not record that this illegal and invalid formula was employed. Baptismal certificates may then subsequently be issued certifying that N or M was baptised "according to the rites of the Church of England", when this will be untrue.
Former Anglicans need no 'rebaptism' when there is evidence that the Sacrament was validly administered in accordance with the rites and ceremonies authorised in the Church of England. But I believe that a mere certificate of baptism is no longer adequate proof of this, and that when this is the only evidence provided, Baptism should be administered to a convert conditionally.
A safe rule of thumb would be to apply this praxis to Anglican baptisms done later than, say, 2000. Or 1990?