How to implement a singleton

I have made a script with a counter in it, and then to make this counter keep count through the whole game, I am using DontDestroyOnLoad (transform.gameObject);. There is just the problem, that when i start on the first scene nothing is wrong but, then when I have played through a level and is back on the main scene, the counter(which is displayed) has been duplicated. I saw something about implementing a singleton in another thread, but I didnt really understand how I would implement it into my script. Do I have to make a seperate script with the singleton in, and then connect it to the script with the counter or what would the best way of doing it be?

Script:

 static var energyObj : int = 5;
 
 var counter : float;
 
 function Awake () {
 DontDestroyOnLoad (transform.gameObject);
 }
 
 function Start()
 {
 counter = 1800;
 }
 
 function Update () {
  
  if (energyObj < 5) {
   counter -= Time.deltaTime;
      if(counter <= 0){
         energyObj += 1;
         counter = 1800;
         Debug.Log("Energy is " + energyObj); 
         }
      }
      
  energyObj = Mathf.Clamp(energyObj, 0, 5);
  
  if (energyObj > 5) {
          energyObj = 5;
     }
  if (energyObj < 0) {
          energyObj = 0;
      }
 }
 
 function OnGUI () {
 
 if (energyObj < 5) {
     var minutes = Mathf.Floor(counter / 60).ToString("00");
     var seconds = (counter % 60).ToString("00");
     GUI.Label(new Rect(100,40,400,40),"To Next: " + minutes + ":" + seconds);
     
     }
 
 }