Generate a unique random 5 digit number

How do I generate a 5 digit number that does not repeat (unique)?

public int number; 

    // Update is called once per frame
    void Update () {
        if(Input.GetKeyDown(KeyCode.A))
        {
          num = Random.Range(10000,99999);
        }
    }

Although a similar question was asked previously, I do not believe it is a duplicate. I am trying to generate a unique random number, instead of 2 numbers repeating.