list.Count problem

My code creates a list that eliminates same triple or higher value.

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 
 public class list : MonoBehaviour
 {
     List<int> onlydouble = new List<int>();
     int listLength;
     int checkdouble;
     int randno;
 
     void Start()
     {
         listLength = 7;
         Gener8only2dupli();
 
     }
     void Gener8only2dupli()
     {
         while (onlydouble.Count != listLength)
         {
             Debug.Log(onlydouble.Count);
             randno = Random.Range(0, 3);
             Debug.Log("randno" + randno);
             //Debug.Log("onlydouble" + onlydouble.Count);
             //for (int j = 0; j < MaxNumbers; j++)
             checkdouble = 0;
             foreach (int value in onlydouble)
             {
                 //Debug.Log("a" + a + "j" + j);
                 if (randno == value)
                 {
                     Debug.Log("incremented");
                     checkdouble +=1;
                 }
 
             }
             if (checkdouble > 2)
             {
                 Debug.Log(randno + "greater than two");
             }
             else
             {
                 onlydouble.Add(randno);
                 Debug.Log("added" + randno);
             }
 
         }
     }
 }
 

My problem is that when Debug.Log(onlydouble.Count); executes it says it has a new element added even thought it did not add a new element. What's the proper way in checking the number of elements in generic list in Unity? Thanks