When updating the ADMX Central Store from Windows 10 to Windows 10 v1511, you may encounter the following error when attempting to view or edit group policies:

Administrative Templates
Namespace ‘Microsoft.Policies.WindowsStore’ is already defined as the target namespace for another file in the store.

File \\domain.fqdn\SysVol\domain.fqdn\Policies\PolicyDefinitions\WinStoreUI.admx, line 4, column 80

A similar problem is referenced in KB 3077013 but basically also applies to this issue:

On a domain controller, using Explorer, navigate to the ADMX Central Store:folder containing SYSVOL\SYSVOL\domain\Policies\PolicyDefinitions

Delete the file WinStoreUI.admx and all occurances of WinSt  ...