input: Write a function to remove duplicate characters from a given string. The function should return a new string with all unique characters. If the input string is empty, the function should 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")=="buferow" 
  assert remove_duplicate_characters("abcd")=="abcd"
  assert remove_duplicate_characters("abcabc")=="abc"
  assert remove_duplicate_characters("")==""
  assert remove_duplicate_characters("aaaaa")=="a"
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