As far as I understand it is if you turn on duplication, it duplicates everything in that folder, newly added or existing. And if you turn it off, it no longer keeps those duplicates. I believe it takes a while for the duplication to happen or for drive space to free up (like 24 hours) after it processes it.
But I'd rather hear from an expert as I'm delving into this whole thing fairly new as well. But learning a lot in the process.