It's not arbitrarily 2x the size, it's the sum of all disks and pools with that share root folder.
If the root folder has the same name, it's contents will be displayed together with all the other same name root folders in the user share tree.
That's a useful bit with user shares, you can have bulk storage and fast access files all displayed in the same tree, and move them as needed. The traditional use was to set a share to cache:yes, that terminology has been replaced with the current 6.12rc series, but the functionality was that new files added to the share would go to the fast cache pool, then overnight the mover would transfer them to the array, which has a much slower write speed. The file still appears in the same path, /mnt/user/sharename/filename regardless of whether the file exists on /mnt/disk1/sharename/filename or /mnt/pool/sharename/filename