First scene loads twice

I'm slowly going insane trying to get my head around the SceneManager.

I have a small basic unity game with a total of 6 scenes. The build order is as follows;

0- MainMenu // Click NewGame to start Level_01.
1- Level_01
2- Level_02
3- Level_03
4- YouWin // This should trigger when player completes last level.
5- YouLose // This triggers when player runs out of time.

However when playing, the scenes load in this order;

0- MainMenu
1- Level_01
2- Level_01 // Repeats Level_01
3 - Level_02
4- YouWin // Never reaches Level_03

I have checked and double checked the build order and that does not appear to be the problem. I am relatively new to unity and coding in general and I have been trying to fix this for days and seem to be getting nowhere. Any help would be appreciated.

Here is the GameManager code:

public class GameManager: MonoBehaviour{

 //count
 public static int currentScore;
 public static int highscore;
 public static int currentLevel;

 //timer
 public float startTime;
 private string currentTime;


 void Update()

 {   
     startTime -= Time.deltaTime;
     currentTime = string.Format("{0:0.0}", startTime);
     if (startTime <= 0)
     {
         startTime = 0;
         //this loads the time out screen if timer reaches 0
         SceneManager.LoadScene(5);
     }
 }

 
 public static void CompleteLevel()
  {
      if (currentLevel < 4)
      {  
          //this loads levels in sequence
          SceneManager.LoadScene(currentLevel += 1);

      
      } else {

          //this is the win settings
          SceneManager.LoadScene(4);
          print("You win");
      }
  }

This is the MainMenu code:

public class MainMenu3 : MonoBehaviour{

 // Loads the levels from clicking (needed for both MainMenu and YouLose scenes)
 public void LoadScene(string name) {

     SceneManager.LoadScene(name); }


 //this quits the game
 public void QuitGame() { 

     Application.Quit();
 }

This is code for triggering next scene when reaching goal:

public class PlayerController2 : MonoBehaviour{

 //player
 private Rigidbody rb;


 void Start ()
 {
 
     rb = GetComponent<Rigidbody>();

 }
     

 void OnTriggerEnter (Collider other)
 {
     if (other.transform.tag == "Goal")
     {
          GameManager.CompleteLevel();
     }   
 }

Any help at all would be hugely appreciated! Thanks for reading.

*Edit: cleaned up code to make it easier to read