Double Jump and Jump both getting executed

Both Jump and Double jump are getting executed, could somebody please help me?

using UnityEngine; using System.Collections;

public class Player : MonoBehaviour {

 public float JumpForce = 5f;
 private bool isJumping = false;
 private bool isJumping2 = false;
 public GameObject PlayerObj;
 public GameObject GroundObj;


 // Use this for initialization
 void Start()
 {

 }

 // Update is called once per frame
 void Update()
 {
     //Jumping Section
     if (Input.GetKeyDown(KeyCode.Space) || Input.GetMouseButton(0))
     {
         if (isJumping == false)
         {
             PlayerObj.rigidbody2D.velocity = new Vector2(rigidbody2D.velocity.x, 0f);
             PlayerObj.rigidbody2D.AddForce(new Vector2(0f, JumpForce), ForceMode2D.Impulse);
             isJumping = true;
             print("Jump");
         }
         else if (!isJumping2)
         {
             PlayerObj.rigidbody2D.velocity = new Vector2(rigidbody2D.velocity.x, 0f);
             PlayerObj.rigidbody2D.AddForce(new Vector2(0f, JumpForce), ForceMode2D.Impulse);
             isJumping2 = true;
             print("Double Jump");
         }

     }
 }

 //this bit is to check if the object that collided with it has the ground tag
 
 void OnCollisionEnter2D(Collision2D other)
 {
     if (GroundObj.gameObject.CompareTag("Ground"))
     {
         isJumping = false;
         isJumping2 = false;
         print("Ground");
     }
 }

}