**Question**

At this moment, I have this:
[![enter image description here][1]][1]

What is the most efficient way to achieve a quad as *cut section*, where

 - all the faces as well as the unselected vertices are duplicated and
   overlapping
 - but share the same vertices (selected).

The *cut section* would look like this:
[![enter image description here][2]][2]

  [1]: https://i.sstatic.net/GPpXR.png
  [2]: https://i.sstatic.net/gP5pt.png