input: Write a function to remove duplicate characters from a given string. If a character occurs more than once in the string, it should only appear once in the result. If the input string is empty, return an empty string.
  assert remove_duplicate_characters("bufferoverflow")=="buferow"
  assert remove_duplicate_characters("aaabbcccbad")=="abc"
  assert remove_duplicate_characters("abcdefgh")=="abcdefgh"
  assert remove_duplicate_characters("")==""
output: def remove_duplicate_characters(s):
      seen = set()
      result = []
      for char in s:
          if char not in seen:
              seen.add(char)
              result.append(char)
      return ''.join(result)