HFC MAC ID already exists

You're out of luck unless your gateway can do "hardware mac spoofing".  Some 3rd party devices can.  Xfinity stuff cannot.  It also doesn't appear their customer support, at least on the levels you can get to can remove an incorrect MAC (or bump someone spoofing), then add you.

New in box:  Since the default action is to replace an Xfinity gateway, you probably have someone else's problem repackaged as 'New' if that is what it is -- you didn't say what the hardware was.  No company on earth could afford absorb and scrap the amount of hardware this company does, even if the offset is a customer engagement opportunity to up-sell services, and that seems to be the business model.

Since it's new, return it, explain the problem is a cloned MAC -- either counterfeit product, database problem, or used resold as new and it won't work in your local cable system.   Pops up with 3rd party gear and online resellers sometimes too.  Your bad luck it was used again on the same system.

They shouldn't give you any grief for a one-for-one swap.

Hi @user_06021b Thanks for bringing your modem concerns to our attention! You have come to the right place for help! I would need access to your account to resolve this. Please send our team a direct message with your full name and full address so that we can assist you further. 

To send a "Peer to peer" ("Private") message:
Click "Sign In" if necessary
• Click the "Peer to peer chat" icon
• Click the "New message" (pencil and paper) icon
• Type "Xfinity Support" in the "To:" line and select "Xfinity Support" from the drop-down list which appears. The "Xfinity Support" graphic replaces the "To:" line
• Type your message in the text area near the bottom of the window
• Press Enter to send it

Concern moved here to the Customer Service help section for greater exposure to actual Comcast corporate employees (The Digital Care Team) for assistance.