MVC: Cannot create/shadow copy 'System.Web.Mvc' when that file already exists

This is a temporary build error. Following steps would help you resolve this error:

1. Wait for some time(30 sec to 1 min).
2. Right click Solution and clean solution.
3. Rebuild Solution.
4. Open some other url in browser bar.
5. Try reloading same url again.

Hope this resolves your error. However if this still does not work for you, then go to temp location (asp.net temp files in program files) and delete all the project related temporary files(dlls). And rebuild solution. You might need to close visual studio as few files could be in use.

This should resolve the error. If you face such errors very frequently, I would recommend you to clear your machine. Delete all the temp files. empty recycle bin, delete unnecessary files from system drive(c drive), clean desktop.

And you still face such errors(and other like visual studio is busy performing some internal operation), in worst case you might need to fix your windows.(if you are unable to do this, get your machine formatted if possible).