input: Write a function to remove duplicates from a given list and return the list with unique elements. If the list is empty, return an empty list.
  assert remove_duplicates([1, 2, 3, 4, 3, 2, 1])==[1, 2, 3, 4]
  assert remove_duplicates([10, 20, 30, 40, 30, 40])==[10, 20, 30, 40]
  assert remove_duplicates(['a', 'b', 'a', 'c', 'c'])==['a', 'b', 'c']
  assert remove_duplicates([])==[]
output: def remove_duplicates(lst):
      return list(dict.fromkeys(lst))