input: Write a function to remove all duplicate elements from a list of integers. The function should return a new list containing only the unique elements in the order they first appeared.
  assert remove_duplicates([1, 2, 3, 2, 4, 5, 6, 3]) == [1, 2, 3, 4, 5, 6]
  assert remove_duplicates([5, 5, 5, 5]) == [5]
  assert remove_duplicates([]) == []
  assert remove_duplicates([1, 2, 3, 4]) == [1, 2, 3, 4]
  assert remove_duplicates([1, 1, 2, 2, 3, 3, 4, 4]) == [1, 2, 3, 4]
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