input: Write a python function to count the frequency of duplicate elements in a list using the Counter class from the collections module.
  assert count_duplicates([1, 2, 2, 3, 4, 4, 4, 5]) == {2: 2, 4: 3}
  assert count_duplicates([10, 20, 20, 30, 40, 40, 40, 50]) == {20: 2, 40: 3}
  assert count_duplicates([1, 1, 2, 3, 3, 3, 4, 5]) == {1: 2, 3: 3}
output: def count_duplicates(lst):
      from collections import Counter
      count = Counter(lst)
      return {k: v for k, v in count.items() if v > 1}