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.
Are you asking hypothetically? In real world examples… Mac addresses are Layer 2, unlike IP addresses which are Layer3. Basically Mac is a hardware address hardcoded to the device and no two devices will have duplicate addresses. that’s how the system is designed. Officially, no two network devices will ever have the same Mac address. I am ignoring the fact that Mac spoofing is a possibility (software duping network Mac addresses, in which case duplicate Macs possible… but that’s not your situation here).
If two devices had the same MAC on the same network , it could lead to L2 loops and possible network congestion. Some devices may not like the duplication at all and freeze services all together. Behaviour is unpredictable as dependent on device firmware.