Why does Update() call itself more than once?

I’m new in unity and developing a cricket game. I have a screen that shows squad selection. It shows images of 15 players, out of which I have to select any 11. I keep on adding the clicked(selected) player in a List. But the problem is that when I click the player once, the size of my list becomes 4 or 5 or even more sometimes.
My script is as follows:

using UnityEngine;
using System.Collections;

public class clickbull : MonoBehaviour {

	// Use this for initialization
	void Start () {
	void Update () {
		if (Input.touchCount == 1)
			Vector3 wp = Camera.main.ScreenToWorldPoint(Input.GetTouch(0).position);
			Vector2 touchPos = new Vector2(wp.x, wp.y);
			if (collider2D == Physics2D.OverlapPoint(touchPos))
				Material newMat = Resources.Load("New Material", typeof(Material)) as Material;
				gameObject.renderer.material = newMat;
Please guide me where am I going wrong?? why is the List size increasing on its own?? How to use this..Please help, its really urgent.

Update calls itself once per frame, a frame last a fraction of a second, so even the quickest touch will last several frames.

Therefore you need to add some safeguard that only permits the action once per touch. Then unlock it once the touch ends.

Update is called every frame, so that script is called a bunch of times every second as long as you have a finger on the screen.

The solution is to replace

if (Input.touchCount == 1)


if(Input.GetTouch(0).phase == TouchPhase.Began)

which checks if the Input started this frame, which is what you’re looking for.

The 0 means to check the phase of the first (index 0) touch input - ie. the first finger that’s on the screen at the moment.