A MAC address isn’t really unique. Each has six octets, of which three refer to the manufacturer. The other three octets have at most 16,777,216 possible values. That seems like a lot but it really isn’t; a MAC is supposed to be unique on a LAN, not globally. Rollovers during manufacturing happen, and collisions are rare but happen once in a while.
Not that the local DHCP servers falling over has anything to do with it…