17th century Europe saw many trials related to Catholic-Protestant disagreements. Was the legal principle of conflict of interest operative at the time, so that for example the same person could not be the source of a denunciation and at the same time a legal expert called upon for an opinion?
I would be particularly interested in the trials of the inquisition in Italy.
Apart from the issue to what extent laws concerning "ambidexterity" (mentioned in the answer) were actually followed, I am also interested in what type of (canon) law existed at the time. The scenario I am thinking of is slightly different from ambidexterity. Suppose person X initiated a denunciation (or in more favorable modern terminology "whistle-blowing") concerning person Y. Can person X himself be appointed to sit as a judge or offer expert opinion on the ensuing case involving Y stemming from X's whistle?
Note. An instance of such conflict of interests dates from 1632-1633. Melchoir Inchofer was chosen as one of a panel of three theologians appointed to assess Galileo's work, the others being Agostino Oreggi and Zaccaria Pasqualigo. In addition to this overt task, there is in the papers of the Congregation of the Index an anonymous denunciation, referred to as EE291 in the literature. The EE291 dates from 1632 and is in Inchofer's hand. One of the key accusations is Galileo's alleged non-conformity to Canon 2 of Session 13 of the Council of Trent. Additional details can be found in this 2018 publication in Foundations of Science.