Short answer. Arresto mayor. Article 260 imposes arresto mayor on the combatants in a duel whenever neither death nor physical injury results — the article specifically says this applies even though no physical injuries have been inflicted at all. Their seconds face punishment too, treated as accomplices in every case.
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 →
What the law says
The seconds shall in all events be punished as accomplices.
Revised Penal Code, Article 260 — Responsibility Of Duellists. Read the full provision →
The two more serious outcomes this article addresses first
Article 260 sets its penalties in a descending order of severity tied to the outcome of the duel. Killing the adversary carries the penalty of reclusion temporal. Inflicting physical injuries only carries whatever penalty is provided for that kind of injury according to their nature — meaning the applicable injury provisions elsewhere in the Code, scaled to how serious the injury actually was, apply instead of a fixed duel-specific penalty.
The residual case: no death, no injury
Your question falls under the article's third scenario: in any other case, the combatants shall suffer the penalty of arresto mayor, although no physical injuries have been inflicted. The article goes out of its way to say this applies even when nobody was hurt at all — simply engaging in the duel under these circumstances is enough to trigger arresto mayor for both combatants, without needing to show any injury resulted.
Both combatants are treated the same way
Article 260 does not single out a winner or a loser when no injury results — it refers to the combatants as a group, meaning both parties to the duel face the same arresto mayor penalty in this scenario. The article's structure ties the penalty to what happened during the duel as a whole, not to which combatant came out ahead when neither death nor injury occurred.
The seconds are punished too, in every case
Article 260's final sentence applies across all three scenarios, including the one where no injuries occur: the seconds shall in all events be punished as accomplices. "In all events" means this is not limited to the more serious outcomes — even where the combatants only face arresto mayor because no injury resulted, their seconds are still liable as accomplices to whatever the combatants are punished for.