2_mikrotiks_with_same_lan_mac_addresses. 
Didn’t you moved backup or config script from one device to another? I guess so. Simply reset the device and take care about what you are loading in.
No I didn’t.
Is your problem solved?
probably reseller did something wrong.
or more likely that refurbished/repaired routers.
never saw anything like, regardless brand for 20 years.
(there was l4 SMC switch once, long ago, but that confirmed production flaw and batch recalled).
Did the reset without defaults helped?