A current evolutionary hypothesis predicts that the most extreme forms of animal weaponry arise in systems where combatants fight each other one-to-one, in duels. It has also been suggested that arms races in human interstate conflicts are more likely to escalate in cases where there are only two opponents. However, directly testing whether duels matter for weapon investment is difficult in animals and impossible in interstate conflicts. Here, we test whether superior combatants experience a disproportionate advantage in duels, as compared with multi-combatant skirmishes, in a system analogous to both animal and military contests; the battles fought by artificial intelligence agents in a computer war-game. We found that combatants with experimentally improved fighting power had a large advantage in duels, but that this advantage deteriorated as the complexity of the battlefield was increased by the addition of further combatants. This pattern remained under the two different forms of advantage granted to our focal A.I. combatants, and became reversed when we switched the roles to feature a weak focal A.I. among strong opponents. Our results suggest that one-on-one combat may trigger arms races in diverse systems. These results corroborate the outcomes of studies of both animal and interstate contests, and suggest that elements of animal contest theory may be widely applicable to arms races generally.