input: Write a function to remove duplicate characters from a given string. Return the modified string without any duplicate characters.
  def remove_duplicate_characters(string):
      result = ""
      seen = set()
      for char in string:
          if char not in seen:
              seen.add(char)
              result += char
      return result
  
  assert remove_duplicate_characters("bufferoverflow")=="bufrOvew")
  assert remove_duplicate_characters("aaabbbccc")=="abc")
  assert remove_duplicate_characters("mississippi")=="misp")
  assert remove_duplicate_characters("a")=="a")
  assert remove_duplicate_characters("")=="")
output: def remove_duplicate_characters(string):
      result = ""
      seen = set()
      for char in string:
          if char not in seen:
              seen.add(char)
              result += char
      return result