Just a random question/thought that popped up in my head: If you had one router with its real mac address connected to the internet on a particular ISP, and you cloned the Mac of another router to make it the same of the 1st one and connected it to the same ISP, would it make the connections for both or just one connection unstable, not work at all or would the ISP-level routing work around it?
As far as I’m aware ISPs (at least Virgin) lease routers their IP address based on their MAC. Essentially the ISP will be trying to lease the same IP to 2 devices at once.
Having dealt with duplicate MACs in the past on a LAN, it’s chaos. Both connections can be affected. If they’re on the same network, it comes down to an ARP table race as to who gets to talk. If there’s enough separation between them that they don’t end up in the same ARP table someplace, it may have no impact at all. It will be similar to an ARP table poisoning attack.