Because your PC would not be able to get on the internet if this was true, my bet is something else is going on. Example. When a PC checks out a LAN IP address the PC might ask the router for that same address and you might get a pop up complaint. There is nothing to fix here if you can get to the internet.
Eventually the message will stop. I don't see any reason to write more about this.
I am on my landlord's WiFi. I have my own password. Today MS popup said that 'there are 2 computers with the same IP address'. Microsoft Essentials says 'PC potentially insecure'. Since this is a shared WiFI, can I change my IP address with out affecting his or without going through him?