I'm guessing the folders are not at the lowest level. All mounts need to be at the lowest folder level to avoid this type of behavior. So your User Documents structure could look like this:
In this scenario, you would want to avoid setting the root User Documents folder to "Documents." Rather, make the root the "(empty)" folder and DEV, TEST, and PROD should be mounts. That should fix the duplication.
That worked like a charm. But I still believe it s a little cumbersome having to set up the user documents this way. I had thought (wrongly, as it turns out) that the root folder meant the root folder of my user documents. However, it looks like the mounted folders and the root folder need not have any relationship at all.