input: Write a python function to remove duplicate characters from a string.
  assert remove_duplicates("aabcdbb") == "acdb"
  assert remove_duplicates("hello") == "heo"
  assert remove_duplicates("testcase") == "tcase"
  assert remove_duplicates("mississippi") == "m"
  assert remove_duplicates("characters") == "chartes"
output: def remove_duplicates(s):
      seen = set()
      result = []
      for char in s:
          if char not in seen:
              seen.add(char)
          else:
              result.append(char)
      return ''.join(char for char in s if char not in result)