input: Write a python function to remove duplicate words from a string.
  assert remove_duplicates("I have to go go shopping shopping") == "I have to go shopping"
  assert remove_duplicates("hello hello world world") == "hello world"
  assert remove_duplicates("this is a test test sentence") == "this is a test sentence"
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)