To challenge another player to a contest of skill in battle, you can issue a duel. Killing a player who you are in a duel with incurs no penalties (though keep in mind, combat of any kind is outlawed in the vicinity of most stations.)

/duel challenge challenges another player to a duel.

/duel accept accepts someone's challenge. The duel begins immediately.

/duel decline declines someone's challenge.

/duel abort requests to call a truce. No victor is recorded.

/duel forfeit forfeits the duel. The other player is recorded as the winner.