From Sklo on the official forums in 2016 (there are so many more posts on this topic but i think this sums it up / search string `wurm unlimited duplication error`)
It is not really a bug in the code, it's an unstable authentication process which happens to people who have a bad internet connection. It occurs when your IP address is changed due to a short disconnect of your internet connection from your provider. This looks like the main reason, I have been able to reproduce this many times. The problem most likely is on your end and I doubt that CC can do anything here as long as they don't recreate the whole authentication process.
One thing you could try is to flush your DNS cache.
exit steam run cmd.exe as admin (just type cmd in the bottom left start>search box and right click run as admin)
then in the window type ipconfig /flushdns you could also try /release and /renew in that order
If one of your accounts keeps getting that message, what fixes it for me is: logging in on another server with that account name, then log out and then try logging into sklotopolis (always works for me for some reason)
🍒Mayor of Cherrytop🍒 (Novus) [04:20:01] You can't bring yourself to smelt the door lock. What's up with you today?