That has to do with the Multi-User feature enabled with JellyBean 4.2 (not 4.1). In order to handle separate accounts, parts of the directory structure had to be changed.
/sdcard/legacy e.g. always points to the currently logged-in user's sd card directory.
I currently cannot find the document where I read the details, so I cannot link any source. But with the above in mind, it's easy to guess that
/sdcard/0 will most-likely be the device owners (or "first users" -- the "first user" always becomed the "device owner", only this user can create additional accounts) sd card, and so on. I will add a link to a source as soon as I can find it.
EDIT: One of the pages I got that idea from is AndroidNext. 1droid gives a different explanation when it comes to sd card:
With Android 4.2, Google introduced multiple users as a new feature. In order to accommodate multiple users, Google is now giving each user a their own folder for storage. If you upgraded to 4.2 from 4.1, then the 4.2 ROM will look for a certain file in /data to determine whether it needs to migrate all of your files to the new multi-user data structure. By default, 4.2 migrates all of /data/media to /data/media/0.
A problem arose though with custom recoveries. A custom recovery retains the /data/media folder during a factory reset. When you factory reset and then boot a 4.2 ROM again, the 4.2 ROM will migrate everything in /data/media again. It will migrate your files every time you factory reset. This multiple migration is what resulted in some people having their files moved to /sdcard/0 or even /sdcard/0/0 etc.
So it has in fact to do with the new Multi-User feature -- one way or the other.