Fix: The Global element 'configuration' has already been declared

Question

I used the second solution of How to resolve "Could not find schema information for the element/attribute "?

I.e. created a XSD with the button

Answer 1

I struggled with this for a while as well. Turns out, my version of the problem originated from the hidden {PROJECTNAME}.SUO file created by Visual Studio.

My guess is, VS will cache the XSD schema associations in this file. The warnings popped up after I changed the target framework, and they disappeared after I deleted the SUO file and restarted VS.