Trying to call OnTriggerEnter2D once but being called multiple times...

I'm new to Unity and I'm trying to get OnTriggerEnter2D(Collider2D col) to work just once when triggered to add to a variable. But when triggered it keep on adding constantly and not stopping when it is not being triggered anymore. Any help would be appreciated!!

This is what what I did for the trigger to be called

 // Use this for initialization
 void Start () {
 
 }
 
 // Update is called once per frame
 void Update () {
 
 }

 public void OnTriggerEnter2D(Collider2D col)
 {
     targetHit ++;
     Resetting.Reset();
     
 }

}

And this is where its going

 public MonsterTarget monTarget;

 public Text scoreText;
 public Text highScoreText;

 public float scoreCount;
 public float highScoreCount;

 public bool scoreIncreasing;

 // Use this for initialization
 void Start () {
     if(PlayerPrefs.HasKey("HighScore"))
         {
         highScoreCount = PlayerPrefs.GetFloat("HighScore");
     }
 }
 
 // Update is called once per frame
 void Update () {
     if (scoreIncreasing)
     {
         scoreCount += monTarget.targetHit;//RIGHT HERE
     }

     if (scoreCount > highScoreCount)
     {
         highScoreCount = scoreCount;
         PlayerPrefs.SetFloat("HighScore", highScoreCount);
     }

     scoreText.text = "Score: " + Mathf.Round(scoreCount);
     highScoreText.text = "High Score: " + Mathf.Round(highScoreCount);

 }

}