|MAC addresses are the Layer 2 addresses of networking hardware. Similar to how Layer 3 addresses (IP addresses) work. |
They are unique to the device from the manufacture - The first 6 characters of the MAC can identify the manufacture and the last 6 of the MAC is individual to the hardware.
If two devices have the same MAC address, a very similar issue would occur as if there were two computers that have the same IP address. Except you wouldn't notice this error at first, it is handled at the switch level (Layer 2) and would not present itself to the operating system. There would be lost packets/network connectivity to one or both computers.