Isn’t a deliberately planned gun duel a mortal sin?
Yes, it is mortally sinful.
https://www.gutenberg.org/cache/epub/35354/pg35354-images.html1436. The Morality of Duelling.—(a) Generally, the duel is mortally sinful. Like ordinary fighting, it is against charity, and in addition it includes a will to kill or gravely injure another, to expose one’s own life or limb to chance, and to usurp the function of the State. This applies to the challenged as well as to the challenger, for one can decline the combat to which one is dared.
(b) Exceptionally, a duel would not be sinful, if it took on the character of a war, or of self-defense against an unjust aggressor. Thus, in order to shorten a war or to lessen the bƖσσdshɛd, it might be lawful to make the whole issue depend on a single combat between the commanders or between champions chosen from opposing armies, as in the case of David and Goliath (I Kings, xvii); but in modern times such a practice has been abandoned. Again, if a person had to choose between certain death, if he refused a duel, and possible death, if he consented to a duel, it would seem that he is in the position of one attacked by an unjust aggressor; but it is not easy to picture such a case as happening in normal conditions.
1438. Penalties against Duelling.—(a) Church law deprives of ecclesiastical burial those who die as the result of a duel, if unrepentant (Canon 1240); it also declares excommunication reserved simply to the Holy See and infamy against duellists and their helpers (Canon 2351). (b) Civil law in English-speaking countries makes duelling a crime. If death results, it is regarded as murder, and the seconds are liable to punishment as accessories.