[Solved, kind of] Assign Multiple Keys for jumping?

EDIT : I solved it, changed:

 myRigidbody.AddForce(new Vector2(0, jumpForce), ForceMode2D.Impulse);

into:

 myRigidbody.velocity = new Vector2(myRigidbody.velocity.x, jumpForce);

and it works now... something is messing with addforce... maybe my poor ground collision detection?

I have this little jump code here:

 if ((Input.GetKeyDown(KeyCode.Joystick1Button0) || 
      Input.GetKeyDown(KeyCode.Space) || 
      Input.GetKeyDown(KeyCode.UpArrow)) && isGround )
    {
      myRigidbody.AddForce(new Vector2(0, jumpForce), ForceMode2D.Impulse);
    }

But if I press multiple keys at once, Space and Up, I jump three times higher :O ... So, what did I do wrong? (Besides everything)

Tried with GetButtonDown but same result... changed by circular-ground-collision check into a box but that didn't helped either...

And here the entire code:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class ControllerInput : MonoBehaviour
 {
     Rigidbody2D myRigidbody;
 
     [Header("Float Variables")]
     [SerializeField] float speed;
     [SerializeField] float jumpForce;
     [SerializeField] [Range (0.0f, 1.0f)] float groundCheckRadiusX;
     [SerializeField] [Range (0.0f, 1.0f)] float groundCheckRadiusY;
 
     [Header("Air Control?")]
     [SerializeField] bool airControl;
 
     [Header("Jump Key")]
     public KeyCode jumpButton;
 
     [Header("Ignore This!")]
 
     [SerializeField] Transform groundCheck;
     [SerializeField] LayerMask whatIsGround;
 
     Quaternion startRotation;
     Quaternion currentRotation;
 
     bool isGround;
 
     private void Start()
     {
         startRotation = transform.rotation;
         myRigidbody = GetComponent<Rigidbody2D>();
     }
 
     private void Update()
     {
         currentRotation = transform.rotation;
 
         if (Input.GetButtonDown("Jump") && isGround )
         {
             myRigidbody.AddForce(new Vector2(0, jumpForce), ForceMode2D.Impulse);
             isGround = false;
         }
 
         if (isGround || airControl)
         {
             myRigidbody.velocity = 
                 new Vector2(Input.GetAxisRaw("Horizontal") * ( speed * Time.deltaTime * 100), 
                 myRigidbody.velocity.y);
         }
 
         if ((currentRotation != startRotation))
         {
             transform.rotation = Quaternion.Euler(0.0f, 0.0f, 0.0f);
         }
     }
 
     private void FixedUpdate()
     {
         isGround = Physics2D.OverlapBox(groundCheck.position, 
             new Vector2(groundCheckRadiusX, groundCheckRadiusY), 0, whatIsGround);
     }
 
     private void OnDrawGizmosSelected()
     {
         Gizmos.color = Color.red;
         Gizmos.DrawWireCube(groundCheck.position, 
             new Vector3(groundCheckRadiusX, groundCheckRadiusY, 0));
     }
 }