New cable modem already registered to another Comcast Account

Hi,

My modem has been having problems for the past few months requiring power resets.  I ordered a brand new replacement modem from Amazon which was a sold directly by Amazon.  It is an Arris SBG7400AC2.  But when I called to activate the MAC address I was told it was already registered to another Comcast customer.  The modem packaging looked factory wrapped new, not factory refurbished.  I don't believe that I was shipped a used modem but it was easier to just contact Amazon and request a replacement.

Well, the replacement came and I was told the same thing by the Comcast phone support rep, the modem MAC address was already registered to another customer.  Can someone at Comcast help me get my new modem registered?  I can privately provide the S/N and MAC.  Thank you.