‘About FBX files, you will find two variants which are ASCII FBX and Binary FBX.’
SketchUp exports FBX as an ASCII FBX file. 3DSMax probably imports both types of FBX files, only the ASCII ones appear as you saw in your case, with triple vertices and doubled faces.
I tested with Blender, and it can’t import ASCII FBX files. So I opened the file in 3D Viewer (Win 10) and saved it as a GLTF Binary file, which I then imported into Blender.
It appeared to me as well as to you, with 24 vertices and 12 faces.
I cleaned it without any problems, so try to find a way to edit it in 3DSMax that doesn’t throw vertices like it did to you.
