Another device on the network is using your IP Address

I have MacOS High Sierra too and call your internet provider and ask for help in getting a new IP address and change your password too and have a lock on it with a password to unlock it if you need too so no one can get into using your internet and after you change everything and I put my password on my refrigerator so I can get to it easier. I have EarthLink as my internet provider and I pay less then $30.00 dollars a month for 24/7 services and 75 Mbpsi, but I have to do all of the wiring by myself which is very easy to do by listening to one of the Earthlink customer service person. Plug the cord into the router and wall outlet and into your computer that is all the wiring you need to do by yourself and it is easy to do. AT&T is $50.00 a month for the same I have at Earthlink, but they will hook up everything for you instead of you doing it yourself like Earthlink wants you to do. I started with EarthLink in 1994 and it was $9.95 a month for internet services for 24/7.

Make sure all your devices connecting to the LAN are using DHCP to obtain their IP Address. Make sure the scope of your DHCP is large enough to handle all your devices.

Generally it happens because some device has a Static IP set, as Bob says, make certain they use DHCP.

The issue is not the public IP Address provided by his ISP. The issue is two devices on his local network are using the same IP Address. This has nothing to do with his ISP.