Short answer. Reclusión temporal. Article 260 imposes that penalty on any person who kills his adversary in a duel. If instead only physical injuries are inflicted, the penalty for those injuries applies according to their nature; even where no injury results at all, the combatants face arresto mayor.
What the law says
The penalty of reclusion temporal shall be imposed upon any person who shall kill his adversary in a duel.
Revised Penal Code, Article 260 — Responsibility Of Duellists. Read the full provision →
What the law says
If he shall inflict upon the latter physical injuries only, he shall suffer the penalty provided therefor, according to their nature.
Revised Penal Code, Article 260 — Responsibility Of Duellists. Read the full provision →
What the law says
In any other case, the combatants shall suffer the penalty of arresto mayor, although no physical injuries have been inflicted.
Revised Penal Code, Article 260 — Responsibility Of Duellists. Read the full provision →
The penalty for killing your adversary
Article 260 sets the penalty for the most serious outcome of a duel directly: the penalty of reclusion temporal shall be imposed upon any person who shall kill his adversary in a duel. This is a fixed penalty tied specifically to killing an opponent within the context of a duel, distinct from how homicide or murder might otherwise be punished outside that specific dueling context.
The penalty scales down for injuries only
Where the duel results in something less than death, the article scales the penalty down accordingly. It provides that if he shall inflict upon the latter physical injuries only, he shall suffer the penalty provided therefor, according to their nature. So instead of the fixed reclusion temporal penalty for killing, a duelist who only injures the opponent faces whatever penalty applies to that specific kind and severity of physical injury under the Code's general injury provisions.
Even an uneventful duel isn't free of penalty
The article does not let a duel that produces no injury at all go unpunished, either. It states that in any other case, the combatants shall suffer the penalty of arresto mayor, although no physical injuries have been inflicted. So even if the duel ends without anyone being killed or hurt, the mere fact of having fought the duel still carries a penalty — arresto mayor — under this article, separate from whatever harm actually resulted from the encounter.
The seconds are punished too
The article also reaches beyond the two duelists themselves. It closes with the rule that the seconds shall in all events be punished as accomplices. A second is someone who assists or represents a duelist in arranging or carrying out the duel, and the article makes clear that this role carries criminal responsibility as an accomplice regardless of how the duel itself turns out — whether it ends in death, injury, or neither.