C6250 MAC addressed already used by someone else.

I just got a new Netgear C6250 modem router from Amazon. When I contacted Xfinity to set it up, they said there is another account that is using the MAC address associated with my device. They said I need to contact that person and have them remove the device from their account. Two guesses on how this is happening: 1) Two devices were given the same MAC address or 2) this unit was sold, set up the owner, returned and resold to me.

Any thoughts on why this is happening and if there's anything I can do other than return it and request a replacement?