The Empire generally takes a dim view on casual killings among its citizens. One exception to this is the Imperial Duel which is used among those of the noble Houses to settle matters of honor and prestige.
In order to avoid problems with the Imperial Guard, a duel must conform to certain rigid standards, including the presence of a judge, seconds, witnesses, and a formal agreement between the parties as to how the outcome of the duel is to be decided (as not all duels are fought to the death).
Typically, a Dueling Square will be drawn by the judge, or one of the witnesses. This is an even square, measuring seventeen paces on a side, within which the contestants must fight the duel.
Should one of the contestants violate the terms agreed upon in the duel, or otherwise disgrace or dishonor the prinicples of the event, it presumably falls to the judge to determine the winner, or to report the violation to the proper authorities for prosecution. This would presumably be corraborated by the witnesses.