[Conroy]

pawn Код:

new FactionHasOwner;
for(new i; i <= MAX_PLAYERS; i++) {
    if(IsPlayerConnected(i)) {
        if(PlayerInfo[i][LeaderOfFamily]) {
            FactionHasOwner = 1;
        }
    }
}
if(FactionHasOwner) return SendClientMessage(playerid, COLOR_RED, "ERROR: This faction already has a leader.");

Use that in the command which gives the leader.