There's still plenty of time on this and hardware isn't final so things might change, but one thing to keep in mind though is that this device creates interesting camera considerations that never had to be looked at in the past.
One cannot see the screen without opening the device. So in order to get an experience similar to what people are used to, they would have to manipulate the device in some manner (open it up). I hear a lot of talk about the lack of an external camera. Of course, where's, for lack of a better term, your view finder? If one wants to see what they're taking a picture of, making sure everything is in frame, etc., they're already going to have to open the device. It's the one drawback to the Duo's design.
But, with this in mind, it's possible for the Duo to run a single camera that is used for both forward facing and rear facing scenarios. Open the device and have the camera pointing towards the users for video conferences and selfies. Have the display panel with the camera folded back and one now has a rear facing camera.
There's then some trade-offs with this design. On one hand, the Duo only really needs 1 camera. The device's posture just dictates whether it's a rear facing or front facing camera. This means that Microsoft saves on the number of cameras the device needs and can either sell the device for a little less or re-invest that into another part (more memory, better camera for the one that remains, etc.). Additionally, if the one camera's quality is high, that means the front and rear facing cameras are high quality. The downside though is the device cannot simply be pulled from the pocket and used as a camera without manipulating the device's posture.
This is also an area where I think it might impact Microsoft's final design. I wouldn't be surprised at all if they increase the thickness of the device just a tad so they can get a higher quality camera in the device. Given how big the camera quality is for many mobile device users, it can't be a sub-par camera.