My double jump script is not working

The problem is likely that you check the Jump button again immediately after jumping. Since its still the same frame as before, since its the same method, the jump button is still pressed so you double jump immediately.

Your code needs to make sure this isn't all happening in the same frame like so:

 var jump : float = 10.0;
 
 var dJump : float = 7.0;
 
 var gravity : float = 20.0;
 
 private var canDJump : boolean = false;
 
 // These two variables become unnecessary
 //private var isJumping : boolean = false;
 //private var isDJumping : boolean = false;
 
 private var velocity : Vector3 = Vector3.zero;
 
 function Update() 
 {
 
     // Note: This should really be done in Start as you don't want to waste time every frame getting the controller
     var controller : CharacterController = GetComponent(CharacterController);
     
     // By moving this above the jump check you can ensure it only happens on frames after the jump was performed
     if (canDJump == true && Input.GetButtonDown ("Jump"))
     {
         velocity.y = dJump;
         //isDJumping = true; // Removed - never used
         canDJump = false; // NEW!
     }
     
     if (controller.isGrounded)
     {
         velocity = Vector3 (Input.GetAxisRaw ("Horizontal"), 0, 0);
         velocity = transform.TransformDirection (velocity);
         velocity *= speed;
         //isDJumping = false; // Removed - never used
      
         if (Input.GetButtonDown ("Jump"))
         {
              velocity.y = jump;
              //isJumping = true; // Removed - never used
              canDJump = true; // NEW!
         }
     }
 
     // This block is flawed - you only want to double jump once right?
     // Then just set canDJump once the second you start jumping
     /*if (isJumping == true)
     {
         canDJump = true;
     }*/
     
     // This block is flawed - just set CanDJump = false the second you start DJumping
     /*if (isDJumping == true)
     {
         canDJump = false;
         isJumping = false;
     }*/
      
     velocity.y -= gravity * Time.deltaTime;
     controller.Move (velocity * Time.deltaTime);
 }