input: Write a function to remove duplicate characters from a given string. If given string is empty then return an empty string as 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
  
  assert remove_duplicate_characters("bufferoverflow")=="buferwolo"
  assert remove_duplicate_characters("aabbccddeeff")=="abcdef"
  assert remove_duplicate_characters("thequickbrownfoxjumpsoverthelazydog")=="thequickbrownfxjmpsvlazydg"
  assert remove_duplicate_characters("")==""
  assert remove_duplicate_characters("abcdef")=="abcdef"
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