I changed the SSID of my home network, and reconnected my Phaser to it. It is now giving me a 'Duplicate IPv6 address' code 018-436. The advice on a previous post is to find and change the address of the duplicate, but I don't think there is one. And anyway I don't know how to interrogate the IPv6 addresses of everything on my home network.
Certainly the two in the printer itself are different. What's more my home router is set to assign unique IP4 addresses to everything on the basis of their MAC address, and as far as I can see, doesn't have a setting for IPv6 internally.
So I don't believe there IS a duplicate device - I think this is some internal issue for the printer, arising when I chaged the SSID name without changing anything else. I've tried resetting the Wi-fi connection from the embedded web server, but only switching from Dual Stack to IPv4 clears the error. Is there a better way?