Could not load file or assembly System.Web.Mvc or one of its dependencies

Question

currently working on asp.net mvc5 (old mvc3 project). It builds just fine, but when I start the project but when I run the project I am facing the following error.

Answer 1

I'm not really sure if this will help. But I also had this message:

Could not load file or assembly 'System.Web.Mvc (1)' or one of its dependencies ...

I don't really know why but when I check out the bin folder of the web application, some of the DLL files have duplicates like "System.Web.Mvc(1).dll" and so on.

What I did was, I deleted all the duplicate files (file names with '(1)' or '(2)' ... on it), then rebuild the web app then it worked again.