On my periscope model, after I joined the cube and two wedges, I could select vertices and by selecting again could see that there were indeed two there, but I was never able to select both of them and merge them. Ditto with faces. In fact, I could only select edges on the active mesh, so I would only get a single edge and couldn’t merge them.
I was finally able to reduce the total number of verices to 12, but only by block selecting all 3 primitives and removing the duplicates.
Any idea why Blender would react like this?
One possibility may be because of the way I constructed the wedges. I started with a cube, selected a top edge, and moved it down on the Z axis. Ditto with the bottom wedge. This would mean that I actually had 3 edges at that point.
So far it’s a mystery.