[Lordzy]

You could create a variable when player enters the duel and when he dies, give score to the killer.
Example:

pawn Код:

new InDuel[MAX_PLAYERS];
CMD:duel(playerid,params[])
{
   InDuel[playerid]=1;
   return 1;
}

public OnPlayerDeath(playerid,killerid,reason)
{
   if(InDuel[killerid] == 1) //Checking whether the killer is in duel.
   {
      SetPlayerScore(killerid)(GetPlayerScore(killerid)+1; //Gives +1 score.
   }
   if(InDuel[playerid] == 0)
   {
     SendClientMessage(playerid, 0xFF0000, "You Lost!");
   }
   return 1; 
}