MAC address: DE:AD:BE:EF:00:00
That is a locally administered MAC address as indicated by the second-least significant bit in the first octet (0xE = 1110b). As such, it is the responsibility of the user/administrator to make sure it's unique in their network.
AFAIK there's no regulation prohibiting such a practice (there should be) but it may be considered ...
Unless I'm completely mistaken, TCP/IP traffic is still routed to the
actual endpoint via MAC addressing on the network layer, hence the
need for unique MAC addresses.
No, you are mistaken. MAC addressing is used by some data-link protocols to deliver layer-2 frames in the same network. MAC addressing is only ever used and seen on the same layer-2 network. ...