Trying to make double jump but don't know solution

This is my code for what my title says, I can do double jump once on start but then I can’t once I land, I’m new to coding:

public float doubleJump = 0f;

    public Transform groundcheck;
public float groundDistance = 0.4f;
public LayerMask groundMask;

    Vector3 velocity;
    bool isGrounded;

 
    void Update()
    {
        isGrounded = Physics.CheckSphere(groundcheck.position, groundDistance, groundMask);
if(isGrounded && velocity.y < 0)
{
 
    velocity.y = -2f;
}
if(doubleJump > 0 && isGrounded)
        {
              doubleJump = 0f;
        }

Check out a few of these tutorials… you can pick from quite a few ways based on your game’s needs.

sorry but wrong verson