new spawned[MAX_PLAYERS] = 0; // Array 

public OnPlayerSpawn(playerid)
{
      if(spwaned[playerid] == 0) // Check if the player has spawned before
      {
             ShowPlayerDialog(...........);  // Build your dialog here
             spawned[playerid] = 1; 
      }
      //rest of the code
      return 1;
}

// Change the state of that particular playerid in array we created when player disconnects
public OnPlayerDisconnect(playerid, reason) 
{
      if(spwaned[playerid] == 1) 
      {
              spawned[playerid] = 0;
      }
      return 1;
}