OnTriggerEnter Getting called more than once.

Hi All,

I am creating (trying to create) a 2d platformer game. I have a specific type of platform that doubles the players speed when he lands on it. My problem is that the OnTrigger Enter is getting called more than once for the same platform. I have added a boolean to prevent this from happening but surely I shouldn't need this. Can you please tell me why my trigger enter event is getting called more than once.

 using UnityEngine;
 using System.Collections;
 
 public class SpeedController : MonoBehaviour {
         private bool playerIsInTrigger = false;
     private PlayerSettings playerSettings;
     public bool increase = true;
     // Use this for initialization
 
     void Start () {
         playerSettings = (PlayerSettings)GameObject.Find("Player").GetComponent("PlayerSettings");
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 
     void OnTriggerEnter2D(Collider2D other){
         
                 if(playerIsInTrigger)
                     return;
         var speedscale = playerSettings.speedScale;
         if (increase && speedscale <= 1 ) {
             playerSettings.speedScale *= 2f;
         }
         else if (!increase && speedscale >= 1 ){
             playerSettings.speedScale /= 2f;
         }
     }
 
         void OnTriggerExit2D(Collider2D other){
              playerIsInTrigger = false;
          }
         
 }