If that is true then a solution might be to change the MAC address in the original Pi.
But as @flamingm0e says, can this be done in HASSIO?
Also, that wouldn’t explain how the second Pi got its MAC address changed in the first place.
I don’t use HASSIO but I am quite sure it can be done.
Quick search on the web shows someone says this is the way to do it
I reflashed the second Pi with Raspbian and did this but it didn’t work for me.
I seem to be stuck with two Raspberry Pis both with the same MAC address meaning that the a router wants to give them both the same IP address!
Remove your static ip settings in your router relating to your pi. Unplug your original so there is no conflict and see if it comes up as a new device in your router.
I agree with Jim. Unplug both Pi’s and delete the static IP reservations and then reboot your router. Then connect one at a time. There’s no router on earth I know of that won’t let you do this.
The MAC address is usually hard coded in the hardware (although it can be changed with the right tools/software) I have no idea why you are seeing the behaviour you are seeing other than thinking the router is getting confused because you had 2 different Pi’s locked to the same IP address (generally a router won’t let you reserve the same IP address for different devices either)
@DavidFW1960 and @jimpower
I will do this tomorrow, thanks for your input.
In the meantime I found this: (also @zarthan)
Interesting.
I have up to 5 RPi’s on my network at any one time, all wired with no problems, all different MAC and IP addresses.
Ok, so I’ve deleted the static IP reservation (singular) and then added both Pis individually to the network and they do both appear to have the same MAC address.
(My assertion that this Pi had had a different MAC ID previously was wrong. This is a spare Pi I had kicking around and I had got it confused with another one)
This would explain why I only had one IP reservation to delete, the router saw both devices as the same. It also meant that only one would actually connect to the network at once.
I got them from Amazon and as this in my opinion is a manufacturing error I am going to try and get them to accept a return.
Unless of course anyone has any further suggestions?
@jimpower, @DavidFW1960, @zarthan and @anyone_else_who_might_be_reading_this
Did you get them together?
Yeah this is weird I have 6 at my place 3 wired and 3 wireless and have never come across this.
Having on-line chat with amazon now and they will refund for both. Great service fas usual, it’s just a shame I’m encouraging their World Domination 
@DavidFW1960 No I got them all separately which makes it all the stranger. Two were from the same seller though…
So I have 5 Pi’s manufactured in China and the UK as follows:
Pi1B 2 x China, 1 x UK
Pi3B 1 x China (PRC) 1 x UK
Where were yours made? Can you post the MAC address??
Mine all happily coexist on my network. I only use one of the Pi’s wireless, all the rest are wired. Even the wireless one I do use wired from time to time without problems.
So weird!
I have three all wired, the one running KODI has a unique (to me at least!!) MAC address.
You are too late as one is already on its way back to Amazon so I can’t tell you where they were both manufactured I’m afraid. The other one with the same MAC address is my HA server so I will wait until the two new ones that I have ordered arrive before I send that one back.
The offending MAC address is:
B8:27:EB:9E:71:31
Why not just spoof the address as suggested above?
So weird… First 3 are same as both mine B8:27:EB weird.
The first 3 are the manufacturer ID of the network chip 
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As I said it’s too late now as one is on its way back to Amazon but in any case I couldn’t see how I could do it with hassio (and I tried in Raspbian and it didn’t work but that may well be me, I’m not a Linux guy).
But apart from that, call it OCD but I’d rather it was all correct without having to spoof it. If I ever changed the use of the Pi in future I’d have to remember I needed to spoof the MAC address and if that was long time in the future I’d probably forget and then spend hours working out what was wrong!! 
Of course the first 3 are the same, that is the manufacturer id.
2 years ago there are something like 10 million pis had been sold. Their Mac address space is approx 16 million. Each modern pi consumes 2 addresses (one for wifi, one for wired).
Unless they have some additional manufacturer numbers, they ran out of unique mac addresses some time ago.