input: Write a function to remove duplicate characters from a given string.
  
  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") == "bufroew"
  assert remove_duplicate_characters("aaabbcc") == "abc"
  assert remove_duplicate_characters("abcdef") == "abcdef"
  assert remove_duplicate_characters("aabbaaccddee") == "abcd"
  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