I actually noticed the same thing and here are some screenshots of my instance.
Version: 126.96.36.1990 Beta
OS: Windows Server 2012 Essentials
Drives: 2x 2TB in Pool
Duplication Setting: Pool Duplication enabled.
DrivePool screen showing the "x2" is on for the drive pool and 20.2gb are being duplicated as well as 3.56 gb "other"
DrivePool screen showing the same thing when hovering over the drives. Duplicated 10.1gb, 1.78gb other. It shows this for both drives.
DrivePool w/ Drive Details
The propery screen for the hidden folders on each drive: 11.6gb each
It looks like all the files are on both drives so I don't think it's an issue with duplication not working... I think something isn't being reported correctly back to the UI. In the previous version isn't there an option to check file duplication integrity? If not, is that something that can be added down the road?