Parallels has a neat feature to allow you to create a VM from a bare metal install of Windows in the Bootcamp partition (rather than having it installed to a file representing a virtual disk on the host). This allows you to either natively boot straight from the Bootcamp partition, or boot in a VM running on a Mac OS X host.
For Windows 8.x, this worked fine even though Windows Activation saw the bare metal install and when running in the VM as two different installs. Previously one would activate as normal, and the other would require a call to the Microsoft number to get a new activation code. Once you had activated both, then you could boot either and both would be activated from one license.
On Windows 10 however, it looks like which ever you boot second, it sees the activation code already used on one of your Windows 10 devices, and then refuses to activate. This is discussed in this Parallels forum post here. So far it seems if you leave Windows 10 booted for ‘long enough’ eventually it will activate itself? I’m having this issue, so leaving my unactivated native boot up and running for a while to see whether it activates or not.