Question: When running a web application project, at seemingly random times a page may fail with a CS0433 error: type exists in multiple DLL's. The DLL's are all generated DLL's residing in the "Temporary ASP.NET Files" directory.

Answer: 
Add the batch="false" attribute to the "compilation" element of the web.config file.

This problem occurs because of the way in which ASP.NET 2.0 uses the application references and the folder structure of the application to compile the application. If the batch property of the element in the web.config file for the application is set to true, ASP.NET 2.0 compiles each folder in the application into a separate assembly.

[http://www.sellsbrothers.com/1995](http://sellsbrothers.com/1995)

<http://support.microsoft.com/kb/919284>
App\_Code folder is causing the problem , put the class outside the folder (Works fine)

App\_Code folder is not designed for Web Application Projects 

<http://vishaljoshi.blogspot.in/2009/07/appcode-folder-doesnt-work-with-web.html>