Change the value to something unique? Of what part of the xml?
Like it says, the value of Prefix to something not already used. That's easy enough, since the base game just uses s1, s2, s3, etc. So you'd change this:
to something like this:
Then go through and replace any occurence of the original name to your new one. So if you made yours a copy from Arcean style, you'd replace anywhere in the xml where it says "Arcean Empire Style" with "Bob's New Style".