Here's a nifty new feature in the dashboard for everyone that manages multiple organizations.
No matter which organization you are logged in to, hit the / key and you can type in a MAC address and it will show you if any devices match that address, across all of the organizations that you have access to. And you can click on the device and it will take you right to it, even if it is in another organization that you are not currently looking at.
This is a great feature for MSP partners.
Hopefully only one device will show up.
I have seen on really large (1000+) Cisco phone deployments in the past where two phones had the same MAC address. This was a real pain to hunt down with traditional Cisco routers and switches.