Duplicates.cs

  int[] arr = { 1, 2, 3, 4, 5, 6, 3, 2, 1 };
            int[] duplicates = new int[arr.Length];
            for (int j = 0; j < arr.Length; j++)
            {
                int counter = 0;
                for (int i = 0; i < arr.Length; i++)
                {
                   
                    if (arr[j] == arr[i])
                    {
                        counter++;
                        if (counter > 1)
                        {
                            duplicates[i] = arr[i];
                        }
                    }
                }
              
            }
            WriteLine(string.Join(", ", duplicates));