input: Write a python function to remove duplicates of characters in a given string.
  assert remove_duplicates_from_string('hello world!') == 'helo wrd!'
  assert remove_duplicates_from_string('aabbcc') == 'abc'
  assert remove_duplicates_from_string('abcabc') == 'abc'
  assert remove_duplicates_from_string('') == ''
  assert remove_duplicates_from_string('a') == 'a'
output: def remove_duplicates_from_string(s):
      seen = set()
      result = []
      for char in s:
          if char not in seen:
              seen.add(char)
              result.append(char)
      return ''.join(result)