input: Write a function to remove duplicate characters from a given string and return the resulting string. If the string is empty, return an empty 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("Hello")=="Helo"
  assert remove_duplicate_characters("Python")=="Python"
  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