There are still problems with the database on the Windows 10 Store where it won't show something as already owned. This is on Microsoft's end, and it is up to them to get it fixed before 10 goes live for that device, as in getting info fixed for phone purchases before Windows 10 for Mobile goes live, and getting fixed for PCs when that OS version goes live, and I suppose another one for Xbox.
This is supposed to be a known issue.