input: Write a python function to remove duplicate words from a sentence.
  assert remove_duplicates("this is is a test test sentence") == "this is a test sentence"
  assert remove_duplicates("hello hello world world") == "hello world"
  assert remove_duplicates("no duplicates here") == "no duplicates here"
output: def remove_duplicates(sentence):
      words = sentence.split()
      unique_words = []
      for word in words:
          if word not in unique_words:
              unique_words.append(word)
      return ' '.join(unique_words)