Speaking of unique identifiers: when I was like 13, my parents bought new ethernet cards so that all the kids would have their own computer. Two adjacent ones on the shelf that happened to have the same MAC address. Not sure how they figured it out (probably just looking for anything wrong and noticing the MAC addresses were identical), but the network behavior was interesting. Basically if you mashed F5 on a webpage…
Duplicate MACs were (well, still are) a huge problem in places with multiple on prem VM pods with shared networks. It's one of those things that seems automatically handled until you realize it's not ALL automatically handled for you.