input: Write a function to remove duplicate characters from a given string. The function should return a new string with all unique characters in the order they first appear.
  
  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")=="bufrloew"
  assert remove_duplicate_characters("mississippi")=="misp"
  assert remove_duplicate_characters("aabbccdd")=="abcd"
  assert remove_duplicate_characters("abcdef")=="abcdef"
  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