From your description it sounds like they are cloning a modem with a static IP. It would only work with a business customer that wouldn't really notice the higher volume of traffic until they were contacted by the MSO about going over thier usage limit. But it would still cause a conflict occasionally when there were too many users on at the same time.
Never argue with an idiot, they will drag you down to their level and beat you with experience. Winston Churchill
This is CABL.com posting #341529. Tiny Link: cabl.co/mbA0H