Reset another player at will

When you trade/duel request another player, that player is set as your recipient.

player.getTrade().setTradeRecipient(affectedPlayer);

If you then do any action to cancel the trade request (e.g. walk, trade someone else) this is called:

public void resetAll() {
    Player tradeRecipient = getTradeRecipient();
    if (tradeRecipient != null && !tradeRecipient.isFollowing()) {
        setTradeRecipient(null);
        tradeRecipient.resetAll(); // this should be tradeRecipient.getTrade().resetAll()
    }
    if (isTradeActive()) {
        ActionSender.sendTradeWindowClose(player);
    }
    setTradeActive(false);
    setTradeAccepted(false);
    setTradeConfirmAccepted(false);

    resetOffer();
}

Even with the above line changed, this still allows you to reset another player's trade or duel (explains the dueling bugs observed in the past).