Objection 1. It would seem that a man may not reveal what he knows through confession and through some other source besides. For the seal of confession is not broken unless one reveals a sin known through confession. If therefore a man divulges a sin which he knows through confession, no matter how he knows it otherwise, he seems to break the seal.
Objection 2. Further, whoever hears someone's confession, is under obligation to him not to divulge his sins. Now if one were to promise someone to keep something secret, he would be bound to do so, even if he knew it through some other source. Therefore a man is bound to keep secret what he knows through the confession, no matter how he knows it otherwise.
Objection 3. Further, the stronger of two things draws the other to itself. Now the knowledge whereby a man knows a sin as God knows it, is stronger and more excellent than the knowledge whereby he knows a sin as man. Therefore it draws the latter to itself: and consequently a man cannot reveal that sin, because this is demanded by his knowing it as God knows it.
Objection 4. Further, the secrecy of confession was instituted in order to avoid scandal, and to prevent men being shy of going to confession. But if a man might say what he had heard in confession, though he knew it otherwise, scandal would result all the same. Therefore he can nowise say what he has heard.
On the contrary, No one can put another under a new obligation, unless he be his superior, who can bind him by a precept. Now he who knew of a sin by witnessing it was not bound to keep it secret. Therefore he that confesses to him, not being his superior, cannot put him under an obligation of secrecy by confessing to him.
Further, the justice of the Church would be hindered if a man, in order to escape a sentence of excommunication, incurred on account of some sin, of which he has been convicted, were to confess to the person who has to sentence him. Now the execution of justice falls under a precept. Therefore a man is not bound to keep a sin secret, which he has heard in confession, but knows from some other source.
I answer that, There are three opinions about this question. For some say that a man can by no means tell another what he has heard in confession, even if he knew it from some other source either before or after the confession: while others assert that the confession debars him from speaking of what he knew already, but not from saying what he knew afterwards and in another way. Now both these opinions, by exaggerating the seal of confession, are prejudicial to the truth and to the safeguarding of justice. For a man might be more inclined to sin, if he had no fear of being accused by his confessor supposing that he repeated the sin in his presence: and furthermore it would be most prejudicial to justice if a man could not bear witness to a deed which he has seen committed again after being confessed to him. Nor does it matter that, as some say, he ought to declare that he cannot keep it secret, for he cannot make such a declaration until the sin has already been confessed to him, and then every priest could, if he wished, divulge a sin, by making such a declaration, if this made him free to divulge it. Consequently there is a third and truer opinion, viz. that what a man knows through another source either before or after confession, he is not bound to keep secret, in so far as he knows it as a man, for he can say: "I know so end so since I saw it." But he is bound to keep it secret in so far as he knows it as God knows it, for he cannot say: "I heard so and so in confession." Nevertheless, on account of the scandal he should refrain from speaking of it unless there is an urgent reason.
Reply to Objection 1. If a man says that he has seen what he has heard in the confessional, he does not reveal what he heard in confession, save indirectly: even as one who knows something through hearing and seeing it, does not, properly speaking, divulge what he saw, if he says he heard it, but only indirectly, because he says he has heard what he incidentally saw. Wherefore he does not break the seal of confession.
Reply to Objection 2. The confessor is not forbidden to reveal a sin simply, but to reveal it as heard in confession: for in no case is he allowed to say that he has heard it in the confessional.
Reply to Objection 3. This is true of things that are in opposition to one another: whereas to know a sin as God knows it, and to know it as man knows it, are not in opposition; so that the argument proves nothing.
Reply to Objection 4. It would not be right to avoid scandal so as to desert justice: for the truth should not be gainsayed for fear of scandal. Wherefore when justice and truth are in the balance, a man should not be deterred by the fear of giving scandal, from divulging what he has heard in confession, provided he knows it from some other source: although he ought to avoid giving scandal, as far as he is able.
Source: Summa Theologica (New Advent)