That's correct, because only activity from the current AC month counts. Let's say you started a thread in June which had three comments, and submitted those comments for the June AC. If you then continued that thread in July for another six comments, you can submit the thread starting from those July comments, and the six comments you made in July would all count towards July AC.

There is no overlap between comments made in different months, so at no point can the same activity be submitted twice.