.NET c#随机生成8个数字要求八个数字在1-21之间,同一个数字不能重复出现三次以上(Random)

while (true)
{
     Random r = new Random();
     int[] array = new int[8];
     int[] arr = new int[4];
     for (int i = 0; i < arr.Length; i++)
     {
         arr[i] = r.Next(1,21);
         if (i > 0)
         {
             for (int j = 1; j < i; j++)
             {
                 if (arr[i] == arr[j])
                 {
                     i--;
                 }
             }
         }
     }
     for (int i = 0; i < array.Length; i++)
     {
         int index = r.Next(4);
         array[i] = arr[index];
         int count = 0;
         if (i > 0)
         {
             for (int j=0;j< i;j++)
             {
                 if (array[i] == array[j])
                 {
                     count++;
                     if (count == 2)
                     {
                         i--;
                     }
                 }
             }
         }
     }
     for (int i = 0; i < array.Length; i++)
     {
         Console.Write(array[i] + " ");
     }
     Console.ReadLine();
 }