input: 
  def delete_duplicates(numbers: List[int]) -> List[int]:
      """ Delete all duplicates in a given list of numbers.
      >>> delete_duplicates([1, 2, 3, 2, 4, 1])
      [1, 2, 3, 4]
      >>> delete_duplicates([5, 5, 5, 5])
      [5]
      >>> delete_duplicates([])
      []
      """
  
output:     seen = set()
      result = []
      for num in numbers:
          if num not in seen:
              seen.add(num)
              result.append(num)
      return result