Duplicate MAC addresses

We have a Sophos XG A/P HA cluster. One of the devices shows the following MAC addresses in syslog:

eth0: (PCIe:2.5Gb/s:Width x1) 7c:5a:1c:xx:xx:07
eth1: (PCIe:2.5Gb/s:Width x1) 7c:5a:1c:xx:xx:04
eth2: (PCIe:2.5Gb/s:Width x1) 7c:5a:1c:xx:xx:05
eth3: (PCIe:2.5Gb/s:Width x1) 7c:5a:1c:xx:xx:06

But the active interfaces show MAC addresses like 00:e0:20:xx:xx:xx, which I assume are used because I have HA set up and so a MAC address is generated for the cluster rather than choose one of the nodes MAC address for that interface.

How are these MAC addresses generated? What are the chances of two Sophos devices coming up with the same MAC address? Looking at a small selection of other devices I am already logged in to, i'm guessing the chance is pretty high. One device has a MAC address that differs from my problematic device by 1 bit.

My reason for asking is that we set up a new WAN connection and it dropped packets all over the place. At first I thought it was a duplicate IP address (we were assigned the same IP as another customer), but now I think I was one layer off. There were a bunch of diagnostics I did which I now see were very consistent with the problem being a duplicate MAC address, and by overriding the MAC address (changing 00:e0:20:xx:xx:2a to 00:e0:20:xx:xx:69) all the problems went away.

I'm guessing that there was another customer somewhere experiencing the same problem as us!

The ISP delivers WAN via IPoE (over a VDSL link) on a /24. We can't actually see other customers MAC addresses within the /24, but obviously with the way they have it set up, a duplicate MAC addresses confuses it.

James