Destroy question if its already asked

Hello Everyone, I made a Quiz Game thanks to the Unity Tutorial https://www.youtube.com/watch?v=hLNPjIxf0C8.

Now I have put this code 'int questionIndex = Random.Range(0, questionPool.Length -1);' in the script under 'private void ShowQuestion()'

To pick up random questions, it works. But its take the same questions that is just asked. I want it to go like question is asked and don't come back in the round and go on with the new questions I am 3 days trying to fix this problem but nothing. I hope you guys can help me out.

Here is the script

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 using UnityEngine.SceneManagement;
 using System.Linq;
 
 public class GameController : MonoBehaviour
 {
 
     public int playerHealth = 30;
     int damage = 10;
     public Scrollbar HealthBar;
     public float Health = 30;
 
     public AudioClip Goed;
     public AudioClip Fout;
     public AudioClip klikklak;
 
     public Text TotalscoreDisplayText;
     public Text questionDisplayText;
     public Text scoreDisplayText;
     public Text timeRemainingDisplayText;
 
     public SimpleObjectPool answerButtonObjectPool;
     public Transform answerButtonParent;
 
     public GameObject questionDisplay;
     public GameObject roundEndDisplay;
 
     public Text HighScoreText;
 
     AudioSource GOED;
     AudioSource FOUT;
     AudioSource KlikKlak;
 
     private DataController dataController;
     private RoundData currentRoundData;
     private QuestionData[] questionPool;
 
     private bool[] alreadyAskedQuestions;
 
     private bool isRoundActive;
     private float timeRemaining;
     private int questionIndex;
     private int playerScore;
 
     private List<GameObject> answerButtonGameObjects = new List<GameObject>();
     
 
     void Awake()
     {
         GOED = GetComponent<AudioSource>();
         FOUT = GetComponent<AudioSource>();
         KlikKlak = GetComponent<AudioSource>();
     }
 
     // Use this for initialization
     void Start()
     {
         dataController = FindObjectOfType<DataController>();
         currentRoundData = dataController.GetCurrentRoundData();
         questionPool = currentRoundData.questions;
         UpdateTimeRemainingDisplay();
 
         playerScore = 0;
         questionIndex = 0;
 
         ShowQuestion();
         isRoundActive = true;
 
         HighScoreText.text = " " + ((int)PlayerPrefs.GetFloat("HighScore")).ToString();
     }
 
     private void ShowQuestion()
     {
 
         RemoveAnswerButtons();
 
         int questionIndex = Random.Range(0, questionPool.Length - 1);          
         
         QuestionData questionData = questionPool[questionIndex];
         questionDisplayText.text = questionData.questionText;
 
         for (int i = 0; i < questionData.answers.Length; i++)
         {
             GameObject answerButtonGameObject = answerButtonObjectPool.GetObject();
             answerButtonGameObjects.Add(answerButtonGameObject);
             answerButtonGameObject.transform.SetParent(answerButtonParent);
 
             AnswerButton answerButton = answerButtonGameObject.GetComponent<AnswerButton>();
             answerButton.Setup(questionData.answers[i]);
 
             timeRemaining = currentRoundData.timeLimitInSeconds;
         }
     }
 
     private void RemoveAnswerButtons()
     {
         while (answerButtonGameObjects.Count > 0)
         {
             answerButtonObjectPool.ReturnObject(answerButtonGameObjects[0]);
             answerButtonGameObjects.RemoveAt(0);
         }
     }
 
     private void UpdateTimeRemainingDisplay()
     {
         timeRemainingDisplayText.text = "TIJD: " + Mathf.Round(timeRemaining).ToString();
     }
 
 
 
     public void AnswerButtonClicked(bool isCorrect, bool isNotCorrect, float value)
     {
         if (isNotCorrect)
         {
             playerHealth -= damage;
             FOUT.PlayOneShot(Fout);
             Health -= value = 10;
             HealthBar.size -= Health / 40f;
             if (Health == 10)
             {
                 KlikKlak.PlayOneShot(klikklak);
             }
         }
         if (isCorrect)
         {
             GOED.PlayOneShot(Goed);
             playerScore += currentRoundData.pointsAddedForCorrectAnswer;
             scoreDisplayText.text = "SCORE: " + playerScore.ToString();
             TotalscoreDisplayText.text = "Totaal " + playerScore.ToString();
         }
         if (questionPool.Length > questionIndex + 1)
         {
             questionIndex++;
             ShowQuestion();
         }
         else
         {
             EndRound();
         }
     }
 
     public void EndRound()
     {
         if (PlayerPrefs.GetFloat("HighScore") < playerScore)
             PlayerPrefs.SetFloat("HighScore", playerScore);
         isRoundActive = false;
         questionDisplay.SetActive(false);
         roundEndDisplay.SetActive(true);
     }
 
     public void ReturnToMenu()
     {
         SceneManager.LoadScene("MenuScreen");
     }
 
     // Update is called once per frame
     void Update()
     {
         if (isRoundActive)
         {
             timeRemaining -= Time.deltaTime;
             UpdateTimeRemainingDisplay();
             if (timeRemaining <= 0f)
             {
                 EndRound();
             }
             else
             if (playerHealth <= 0)
             {
                 playerHealth = 0;
                 EndRound();
             }
         }
     }
 }