Multiple duplicate devices.

I am having same problem. One night an unknown device tried connecting to my network. Arris. I looked on my xfinity app nothing about it couldnt find it. Then i got looking in the. Settings and a subnet mask whatever that is was set up. Then i used an app to see who is on my wifi....omg there is a Naray communications.....but dont show up on my xfinity app. I then used admin tool and see duplicate mac addresses....the MoCA keeps getting enabled. I never set up subnest mask. I never set up a Naray thing. I been doing a little noodling around and believe my internet has somehow been used to give others in my apartment building their wonderful gaming pleasures or something.