Movement speed doubled when moving diagonally

Hey everyone,

I have been having a problem with movement of a sphere; the speed is doubled when moving in a diagonal direction (WASD). I have read up on this and the issues stems from the force being applied twice when the 2 directions are pressed, ie. W +D.

The information I have come across suggests using .Normailze.
However I have added this to my code and there has been no resolution. Could anybody give me any advice?

  public float maxSpeed = 5f;
  public float Speed;
  private Rigidbody rb;

  void Start ()
   {
     spawn = transform.position;
     rb = GetComponent<Rigidbody>();
   }

 Vector3 direction = new Vector3(0, 0, 0);

     float moveHorizontal = Input.GetAxis("Horizontal");
     float moveVertical = Input.GetAxis("Vertical");

     Vector3 movement = new Vector3(moveHorizontal, 0.0f, moveVertical);
     direction.Normalize();

 if (rb.velocity.magnitude < maxSpeed)
     {
             rb.AddForce(movement * Speed);
     }

Thanks for Reading, any help would be appreciated