input: Write a function to remove duplicates from a list and return the list in the same order as they appear. 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([1, 1, 1, 1]) == [1]
  assert remove_duplicates([]) == []
  assert remove_duplicates([5]) == [5]
  assert remove_duplicates([1, 4, 2, 3, 4, 2]) == [1, 4, 2, 3]
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