I’m dealing with some dynamically generated DXF files and ran across this problem. If a file changes on disk and you undo/redo the import, the changed file isn’t imported. The previous file’s content – no longer on the drive – is loaded.
Rhino 5.0.2 (5A865), OS X 10.10.5 (14F27)
Import DXF file
Change DXF file
Undo Import (CMD-Z)
Redo Import (SHIFT-CMD-Z)