You can only have one control on a page.
I was adding new MVC controllers to my project (I already have a few working fine) and now my project won't display any public page using an MVC based layout. Regular page templates work fine, but any use of MVC templates including the built-in ones results in the "You can only have one <head runat="server"> control on a page." error message. Of course I deleted any new code from after the error message started appearing, but the error is not going away. Of course I can't find any <head runat="server" code in my project at all.
Okay I figured out what the issues was (sort of). My scaffolding added files under the MVC/Views folder. Removing these fixed the issue. I am not sure why as I was not using any of those, but SF didn't like them being there