3 The Church is the protectress and guardian not only of truth, but also of justice and honor, in the union of which public peace and order are held together; therefore it has vehemently condemned and taken pains to punish with the gravest penalties possible those guilty of private combat. The constitutions of Our predecessor Alexander III, inserted in the books of canon law, condemn and solemnly denounce these private disputes. The Council of Trent punishes with singular and severe penalties those who engage in these contests or in any way participate in them. Indeed, above all other punishments it brands these persons with disgrace; expelled from the bosom of the Church, they are judged unworthy of the honor of ecclesiastical burial if they die in the struggle. Our predecessor Benedict XIV in his constitution of November 10, 1752, Detestabilem, explained in fuller detail the Tridentine sanctions. In most recent times, Pius IX in his apostolic letter, which opens with Apostolicae Sedis and reduces the number of latae sententiae censures, clearly declares that not only those who contend in the duel incur ecclesiastical penalties, but also those who a called patrinos, seconds, and likewise witnesses and accomplices.
Source: Pastoralis Officii (Vatican.va)