new challengedPlayers[MAX_PLAYERS];
new challengers[MAX_PLAYERS];

dcmd_duel(playerid, params[]) {
     new id = strval(params[0]);
     challengers[playerid] = 1;
     challengedPlayers[playerid] = 1;
     
     SendClientMessage(id, 0x0000FF, "You have been challenged to a duel! Type /accept or decline.");
}

dcmd_accept(playerid, params[]) {
     new id = params[0] //Challenger ID.
     if (challengedPlayers[playerid] == 1 && IsPlayerConnected(playerid) && IsPlayerConnected(id)) {
           //Start duel
     }
}

dcmd_decline(playerid, params[]) {
     new id = params[0] //Challenger ID.
     challengedPlayers[playerid] = 0;
     challengers[id] = 0;
     SendClientMessage(id, 0xCC0000, "Your opponent refused your challenge.");
     return 1;
}