input: Write a function to count duplicate elements in a list.
  assert count_dups([1, 2, 2, 3, 4, 5, 5, 5, 6]) == 3
  assert count_dups([10, 20, 30, 40, 50]) == 0
  assert count_dups([1, 1, 1, 1, 1]) == 4
output: def count_dups(lst):
      from collections import Counter
      count = Counter(lst)
      return sum(v - 1 for v in count.values() if v > 1)