How do you double jump in a 2d game

Questions & Answers
1

this is the script I used for the character to move and jump etc. but I cant figure out how to double jump it is really frustrating.

using UnityEngine;
using System.Collections;

[RequireComponent(typeof(PlayerPhysics))]
public class PlayerController : MonoBehaviour {
	
	// Player Handling
	public float gravity = 20;
	public float walkSpeed = 8;
	public float runSpeed = 12;
	public float acceleration = 30;
	public float jumpHeight = 12;
	
	private float animationSpeed;
	private float currentSpeed;
	private float targetSpeed;
	private Vector2 amountToMove;
	
	private PlayerPhysics playerPhysics;
	private Animator animator;
	
	
	void Start () {
		playerPhysics = GetComponent<PlayerPhysics>();
		animator = GetComponent<Animator>();
	}
	
	void Update () {
		// Reset acceleration upon collision
		if (playerPhysics.movementStopped) {
			targetSpeed = 0;
			currentSpeed = 0;
		}
		
		// If player is touching the ground
		if (playerPhysics.grounded) {
			amountToMove.y = 0;
			
			// Jump
			if (Input.GetButtonDown("Jump")) {
				amountToMove.y = jumpHeight;	
			}
		}
		
		// Set animator parameters
		animationSpeed = IncrementTowards(animationSpeed,Mathf.Abs(targetSpeed),acceleration);
		animator.SetFloat("Speed",animationSpeed);
		
		// Input
		float speed = (Input.GetButton("Run"))?runSpeed:walkSpeed;
		targetSpeed = Input.GetAxisRaw("Horizontal") * speed;
		currentSpeed = IncrementTowards(currentSpeed, targetSpeed,acceleration);
		
		// Set amount to move
		amountToMove.x = currentSpeed;
		amountToMove.y -= gravity * Time.deltaTime;
		playerPhysics.Move(amountToMove * Time.deltaTime);
		
		// Face Direction
		float moveDir = Input.GetAxisRaw("Horizontal");
		if (moveDir !=0) {
			transform.eulerAngles = (moveDir>0)?Vector3.up * 180 :Vector3.zero;
		}
		
	}
	
	// Increase n towards target by speed
	private float IncrementTowards(float n, float target, float a) {
		if (n == target) {
			return n;	
		}
		else {
			float dir = Mathf.Sign(target - n); // must n be increased or decreased to get closer to target
			n += a * Time.deltaTime * dir;
			return (dir == Mathf.Sign(target-n))? n: target; // if n has now passed target then return target, otherwise return n
		}
	}
}
2

// Player Handling
public float gravity = 20;
public float walkSpeed = 8;
public float runSpeed = 12;
public float acceleration = 30;
public float jumpHeight = 12;

     bool doubleJumped = false;

<...>

if (playerPhysics.grounded) {
     amountToMove.y = 0;
     doubleJumped = false;
             
     // Jump
     if (Input.GetButtonDown("Jump")) {
            amountToMove.y = jumpHeight;    
     }
}else if(!doubleJumped){
    if (Input.GetButtonDown("Jump")) {
            amountToMove.y += jumpHeight;
            doubleJumped = true;    
    }
}

Should work…