[Schneider]

pawn Код:

new DuelOpponent[MAX_PLAYERS];

When the duel starts you save the playerid of the opponent in the variable of the current player:

pawn Код:

DuelOpponent[playerid] = opponentID;

Then, at OnPlayerDisconnect:

pawn Код:

if(InDuel[playerid == 1)
{
    new opponentID = DuelOpponent[playerid];
    //do all the stuff to 'opponentID' to get him out of the duel, give money, spawn, etc... 
}