Buying upgrade twice while I shouldn't.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;

 public class NewBehaviourScript : MonoBehaviour {
 public float movespeed;
 public int apples;
 public GameObject NewPrefab;
 public int coins;
 public Text cointext;
 public Text appletext;
 public int maxApples;
 public Text Dialogue;
 public Text ButtonTextYes;
 public Text ButtonTextNo;
 public Button NPCYes;
 public Button NPCNo;
 public int upgradeCost;

 void Start () {
     maxApples = 3;
     movespeed = 0.1f;
     coins = 0;
     apples = 0;
     upgradeCost = 15;
     cointext.text = "Coins: " + coins.ToString();
     appletext.text = "Apples: " + apples.ToString();
     Dialogue.text = "";
     NPCYes.gameObject.SetActive(false);
     NPCNo.gameObject.SetActive(false);
     ButtonTextYes.text = "";
     ButtonTextNo.text = "";
 }
 
 void Update () {
    transform.Translate(Input.GetAxis("Horizontal") * movespeed, 0f, Input.GetAxis("Vertical") * movespeed);
 }

 void OnTriggerStay(Collider other) {
     if (other.gameObject.CompareTag("PickUp")) {
         apples = apples + 1;
         appletext.text = "Apples: " + apples.ToString();
         if (apples == maxApples) {
             Destroy(other.gameObject);
         }
     }
 }

 void OnTriggerEnter(Collider other) {
     if (other.gameObject.CompareTag("Drop"))
     {
         coins = coins + apples * 1;
         apples = 0;
         appletext.text = "Apples: 0";
         cointext.text = "Coins: " + coins.ToString();
         Instantiate(NewPrefab, new Vector3(-1.77f, 1f, 3.75f), Quaternion.identity);
     }
     else if (other.gameObject.CompareTag("NPC")) {
         Dialogue.text = "Would you like to purchase this upgrade?";
         NPCYes.gameObject.SetActive(true);
         NPCNo.gameObject.SetActive(true);
         ButtonTextYes.text = "Yes";
         ButtonTextNo.text = "No";
         if (coins >= upgradeCost)
         {
             NPCYes.onClick.AddListener(OnClickYes1);
         }
         else {
             NPCYes.onClick.AddListener(OnClickYes2);
         }  
         NPCNo.onClick.AddListener(OnClickNo);
     }
 }

 void OnTriggerExit(Collider other)
 {
     if (other.gameObject.CompareTag("NPC"))
     {
         Dialogue.text = "";
         NPCYes.gameObject.SetActive(false);
         NPCNo.gameObject.SetActive(false);
     }
 }

 void OnClickYes1() {
         Debug.Log("l");
         coins = coins - upgradeCost;
         maxApples = maxApples + 1;
         cointext.text = "Coins: " + coins.ToString();
         upgradeCost = upgradeCost + 3;
         Dialogue.text = "Enjoy your upgrade!";
 }

 void OnClickYes2() {
     Debug.Log("k");
     Dialogue.text = "You don't have enough coins";
 }

 void OnClickNo() {
     Dialogue.text = "Perhaps another time then.";
 }

}

Whenever I come close to the npc and then buy the upgrade, it buys it once, then the second time twice and so on. Why is this happening?