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.
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.