Welcome to this site.
Things I notice. You have snapping on, and proportional editing. Both I keep off, unless actually needed. Perhaps your new verts were there but snapped to the location of others?
Your transformation orientation is set to ‘normal’ Again I keep mine to Global unless there is good reason to be otherwise which I do not recall being in that lecture.
The highlighted edge, is clearly not the one that the second, right hand stair cut, has gone through. Implying there is an extra geometry overlying that which the highlighted edge is from. To which, the face select dots show by their position, central to their face that they have not been cut by that right hand stair cut. (Ah. Addressing my next paragraph I realise. Its your duplicated loop merely moved, it is not attached to any of the original mesh)
Shift D copying a loop cut will copy those verts, and edges between them, they will not be attached to the rest of the mesh, not produce a cut if moved to another point on the mesh.
As the first loop cut worked, I see no reason why a second would not. Perhaps it is but snapping 'invisibly onto the first?