Well that is sufficient explanation for me.
I ordered two of my Pis from the same seller albeit a short time apart. I guess I just got unlucky.
16,000,000:1, buy the lottery 
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Yes I almost added a comment to that effect at the end of my reply to you!
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Good point. But that’s really bad if they are doing dups on the same production run. That is potentially a real problem.
I suppose I got mine from different suppliers at slightly different times and one China, one UK so I’d really win the lottery if those ones clashed!!!
Ok…
I now have two new RPis.
I put the SD card with my HA setup in one of them, plugged it into the network and guess what?
Same MAC address!
Something is obviously going on here that I am not seeing…
Anyone got any ideas?
EDIT: I booted up the second new one with Raspbian and it has a different unique (to me at least!!) MAC address. Could it really be that I have had three RPis all with the same MAC ID???
EDIT2: So I put the HA SD card in the Pi with the unique MAC address and it showed up on the network with old MAC address. It looks like somehow HA is setting the MAC address in the RPi. Is that possible?
Yes you can set the MAC address in config.txt in the boot partition. But we have been through this earlier in the thread…
Yes I know a MAC address can be spoofed in software, my question was more wondering out loud (because I am pretty sure the answer is ‘no’) if it is at all possible that a MAC address can be ‘hard’ changed in any way. That is the only explanation I can come up with.
I would suggest that you post the question at the raspberry pi forums. I have read where the MAC is not hard coded and is set on first boot by uboot. Since hassio is built by the HA crew, you could also check with the developers on the Discord site. https://discordapp.com/invite/c5DvZ4e