You are right, it is a mistake. Our intention has been to have the chunks be different, as you suggest. Thanks for spotting it!
I would say that ideally your implementation should be able to handle this situation (e.g., you could have the logic of chunks and of files independent, so if two files share the same chunk, you might have both files point to the same chunk object) but it is not a requirement for the homework—if your implementation passes the tests, you are good to go.
We will push the fix asap!