input: Write a python function to remove duplicate words from a string.
  assert remove_duplicates('this string contains duplicate words') == 'this string contains duplicate words'
  assert remove_duplicates('remove remove duplicate duplicate words words') == 'remove duplicate words'
  assert remove_duplicates('hello hello world world') == 'hello world'
output: def remove_duplicates(s):
      words = s.split()
      seen = set()
      result = []
      for word in words:
          if word not in seen:
              seen.add(word)
              result.append(word)
      return ' '.join(result)