Replicating double-sided normals on a cube face, extruding causes inner faces So I noticed this when I created a cube object, extruded one of the faces and it created an inner face in the inside. Which was weird because normally extruding would just moves the face-out and leaves the original vertices without face inside the cube.

So this is what got and see the inside face but also that original cube in the middle has two normals for one face.


I wondering how to replicate that (The double sided normals), literally, how did I do that? I know I must of hit something to do that since I retried it on another cube and the extrude behaves normally.
it's impossible to have two normals per face. (you can check out the documentation of how the mesh datablock is designed)

The only way you can get this situation is if you managed to duplicate the face. (Shift-D for example) So you do have two faces overlapping, facing in different directions. You can verify that by selecting one face and then moving it away, the other one will stay. 

Of course I can't really tell what you did in that particular scene.