Res iudicata is a juridical institution which is essential for the security of juridical traffic, given that it prevents rights from being the object of on-going controversy. One of he ways of arriving at res iudicata in the processes of Canon Law is the double decision. This juridical figure is unique to Canon Law. The purpose of double agreement is to obtain the most just decision possible. Given that in Canon Law no decision can exist which is not, at the same time, in conformity with reality, it means that the truth of the process must coincide with objective truth. The double decision supposes a guarantee of truth, given that the facts of the case are studied by two tribunals of different grades thus guaranteeing, within what is humanly possible, that the decision reached corresponds to the nature of things. The main problem comes from knowing when two decisions conform or agree with each other. What is central at this point is the identity of action. Thus, two decisions agree when they decree the same thing, always provided that the same subjects are in dispute over the same processal object and in virtue of the same causa petendi. This last aspect takes on special significance in marriage cases where the identification of the concrete chapter of nullity coincides with the causa petendi. There could only be conformity between two marriage decisions when they refer to the same chapter of nullity, as it is not sufficient that they simply each decree nullity of the same marriage. The Motu proprio Causas matrimoniales introduced an accelerated process to confirm a pro nullitate decision obtained in the first instance. This summary process raised several problems of process; the juridical or administrative nature of the decree, which confirmed the first instance decision, was particularly discussed. An-other important question arose when the preceding decision was not confirmed and the case passed to a second instance. In this case we had -and still have- a double