Stop Player/User from multi jump in my Unity game - unity3d

Question

Hello and thanks for reading this post.
I'm new to Unity but regardless of this I managed to make a small 2d game. But I ran into a little problem with the jump function.
The player / user shouldn't be able to multi jump in the game.
This is the C# script that controls the player.
using UnityEngine;
using System.Collections;
public class RobotController : MonoBehaviour {
//This will be our maximum speed as we will always be multiplying by 1
public float maxSpeed = 2f;
public GameObject player;
//a boolean value to represent whether we are facing left or not
bool facingLeft = true;
//a value to represent our Animator
Animator anim;
//to check ground and to have a jumpforce we can change in the editor
bool grounded = true;
public Transform groundCheck;
float groundRadius = 0.2f;
public LayerMask whatIsGround;
public float jumpForce = 700f;
// Use this for initialization
void Start () {
//set anim to our animator
anim = GetComponent <Animator>();
}
void FixedUpdate () {
//set our vSpeed
anim.SetFloat ("vSpeed", rigidbody2D.velocity.y);
//set our grounded bool
grounded = Physics2D.OverlapCircle (groundCheck.position, groundRadius, whatIsGround);
//set ground in our Animator to match grounded
anim.SetBool ("Ground", grounded);
float move = Input.GetAxis ("Horizontal");//Gives us of one if we are moving via the arrow keys
//move our Players rigidbody
rigidbody2D.velocity = new Vector3 (move * maxSpeed, rigidbody2D.velocity.y);
//set our speed
anim.SetFloat ("Speed",Mathf.Abs (move));
//if we are moving left but not facing left flip, and vice versa
if (move > 0 && !facingLeft) {
Flip ();
} else if (move < 0 && facingLeft) {
Flip ();
}
}
void Update(){
//if we are on the ground and the space bar was pressed, change our ground state and add an upward force
if(grounded && Input.GetKeyDown (KeyCode.UpArrow)){
anim.SetBool("Ground",false);
rigidbody2D.AddForce (new Vector2(0,jumpForce));
}
}
//flip if needed
void Flip(){
facingLeft = !facingLeft;
Vector3 theScale = transform.localScale;
theScale.x *= -1;
transform.localScale = theScale;
}
}
And here is the Player object and the GroundCheck Object.
How can I stop the player from being able to multijump. So if he press the upArrow key, He will jump and not be able to jump again before he lands.
Thanks for your time and help
Update
If its hard to see the Images here are the Image on Imgur:
http://imgur.com/GKf4bgi,2i7A0AU#0

Answer 1

You can add another Collider to your player GameObject and make it a trigger with Is Trigger option. Use this code to change a flag variable telling if player is on ground:
private bool isOnGround = false;
void OnCollisionEnter2D(Collision2D collision) {
isOnGround = true;
}
void OnCollisionExit2D(Collision2D collision) {
isOnGround = false;
}
Then you can allow jumping only when isOnGround is true.

Answer 2

You could make little game object called groundController, place it under player.
You're setting there bool value for grounded and in code your checking if your controller overlaps with ground.
watch it here for more info: http://youtu.be/Xnyb2f6Qqzg?t=45m22s