It seems that the copy method of the compClass is not udoable, contrary to create, which works fine.
# Does not work! Cannot be Undone!
ui.undo.startBlock("Copy", enable = True)
parent().copy(op("prefab"), name = "Foobar")
ui.undo.endBlock()
# Does work as intendet. Can be undone via CTRL+Z
ui.undo.startBlock("Create", enable = True)
parent().create(baseCOMP, "Foobar")
ui.undo.endBlock()