IPA: /ˈdjuː.ə.lər/
KK: /ˈdjuːələr/
A person who engages in a duel, which is a formal fight between two people, often with weapons, to settle a dispute or defend one's honor.
The two men were known as skilled duellers, often challenging each other to settle their differences.
Dueller → It is formed from "duel" (from Old French *duel*, meaning a fight or combat between two people) and "-er" (meaning a person who). A dueller is a person who engages in a duel.
Think of someone who participates in a 'fight' ('duel') — the '-er' indicates that this person is involved in that action.