input: Write a function to find duplicate words in a string.
  assert find_duplicate_words("Python is a programming language programming used widely") == ["programming"]
  assert find_duplicate_words("this is a test this is only a test") == ["this", "is", "a", "test"]
  assert find_duplicate_words("no duplicates here") == []
output: def find_duplicate_words(s):
      words = s.split()
      word_count = {}
      duplicates = []
      
      for word in words:
          if word in word_count:
              word_count[word] += 1
          else:
              word_count[word] = 1
      
      for word, count in word_count.items():
          if count > 1:
              duplicates.append(word)
      
      return duplicates