The stamp on keys saying “do not duplicate” is not a heed of warning that once it is duplicated it will suddenly explode. It’s actually for the purposes of access control for landlords or for mall administration, controlling the production of its copies and the possibility of crimes thru the copied key.
These stamps are more likely placed on to the key to inform locksmiths and the public that these keys cannot be duplicated without the permission of the owner or manufacturer and these keys are so important, it should not just be passed around so easily. Typically, mail keys, amenity keys or private rooms that contain confidential papers and objects have this stamp on its keys.
Although a warning has already been placed, legally, these warnings are not part of a valid case for suing – in case someone didn’t follow it. So in short, one way or another the stamp will remain but the key will most likely still is duplicated. Unless it will be patented, which costs a lot of money, the duplication is already under legal oath and owners have their rights for suing those who duplicate it.