Initially, the Surface Duo was going to run a new operating system developed in-house by Microsoft (and presumably, if it saw the light, launched under the Windows branding).
Many parts of the UI look almost identical to Windows Phone and fit the general Windows 10 aesthetic, which is not a bad thing, as the phone would've probably been marketed as running a version of Windows.While we can't see it at its full potential due to the fact that it's not running on a dual-screen device, that influence can clearly be seen across the operating system.