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Duels are a way to fight other players with either your own gear (without the risk of losing it) or with pre-set kits so you're all on the same playing field. The winner gets a certificate and their ELO for the kit used (except own gear) gets higher, whereas for the loser, theirs will get lower.
You can either specifically request to duel a player using
/duel <player> [cash bet]or join a queue to duel random players on either of the available kits at
/warp Duels. The queues have a bet of £250, whereas you can choose not to specify a bet if you send a duel request to a player.