Your local network or your Mac may have the wrong addresses configured.
Internet networking involves a lot of alchemy, and I confess to occasionally dropping an eye of newt (or an IP of newt) into a boiling pot to fix problems on my local network. There’s a particular message that macOS displays in limited cases that is perplexing because it’s the sort of low-level bubbling up that Apple generally takes care of. In this case, your Mac alerts you to a problem that may be of your making or might involve your Wi-Fi gateway or broadband modem. That message: “Another device is using your IP address.”
This conflict prevents your computer from accessing some of the local network and from reaching the internet. Here’s why.
Every device that communicates over the internet needs a unique internet protocol (IP) address, a number that is used by routers to package and send data to the right recipient. That’s true on a LAN or within top-level internet data exchanges, whether it’s a $10 million router or an addressable smart lightbulb. When the internet first began its superfast growth over two decades ago, the addresses used came from a relatively small range, using the IP version 4 (IPv4) standard.
Read more at MacWorld.com