The ‘twin object’ in this case, is the dark brown object, that represents one of two arms support a suspended/lifted object (shelf).
The brown object in question, has a ‘lap joint’ cut, on it. I’m aware, after copying and pasting the object, I can use the rotate tool. But another challenge in regards to duplicating the brown piece, that there is a bore hole that measures 5/8" deep, facing the direction of where the suspended object. There’s also a circle hole, representing the ‘marking point’ when the arm is lifted to a 40 degree angle.
If one knows how to replicate the 2nd arm, that will be placed at the pink arrow in the following image, without having to redo bore hole or circular marking hole.
This is the .skp file: 012_missing_faces.skp (770.7 KB)