This error has nothing to do with ip addresses. It’s about the hostname’s computer (and bonjour / netbios). For some reason, it thinks that another device with the same hostname exists on the local network. Best to give your device an actual name under System Preferences > Sharing > Computer name.
Hi Ben, thanks for checking in; yep I think it was caused by the way I was using Roon (going between wired and WiFi too often) but Roon is not to blame! In any case I haven’t had the problem since I posted. I don’t think I did anything to ‘solve’ it but that’s computers!