Upgrade to 4.2 causes CS0433 error

I think it's a bug, ... but I'm not sure, I've never been good at debugging.

but that's how I understand it. like it's trying to read something 2 times that's in engine itself, ...

Yeah, I suspect it's something that has been changed in the way the project is built. One of the reasons I dug around in the references and code was to try and nail down anything on my end that could cause the issue.

Did you fresh install? Try uninstalling old unity first and then installing 4.2 to a blank folder

Yep, that was my second install attempt. After the first one gave me these problems, I uninstalled all unity stuff and rebooted. I also got rid of all of the data in the \\users\\me\\appdata\\roa$$anonymous$$g\\unity folder prior to the new install. The only thing I haven't done is search the registry for unity keys to delete and I prefer not to do that.

Check config.xml for multiple entries of

add assembly="System.Xml.Linq,

Also I can't find System.Xml.Linq.XDocument in any of my PC builds. Is it an addon? Try removing any trace of it if it is.