input: Write a function to remove duplicates from a given list and return the list in the same order. 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([5, 5, 5, 5, 5]) == [5]
  assert remove_duplicates([]) == []
  assert remove_duplicates([1, 2, 3, 4, 5, 6]) == [1, 2, 3, 4, 5, 6]
output: def remove_duplicates(lst):
      seen = set()
      result = []
      for item in lst:
          if item not in seen:
              seen.add(item)
              result.append(item)
      return result