input: Write a function that removes duplicates from a list. The function should return the list with all duplicates removed. If the list is empty, it should return an empty list.
  assert remove_duplicates([1, 2, 3, 4, 3, 2, 1]) == [1, 2, 3, 4]
  assert remove_duplicates([5, 5, 5, 5]) == [5]
  assert remove_duplicates([]) == []
output: def remove_duplicates(lst):
      return list(set(lst))