There is a duplicate ‘system.web.extensions/scripting/scriptResourceHandler’ section defined

Above issue screwed up my head, I spent couple of hours on it and finally I got the solution. Some reasons behind these issues are

1) You might changed “sharepoint – 80” application pool’s .net framework version 2 to 4
2) You might have the web.config file in your site or sub site it contains the duplicate section of scriptresourceHandler, You should retain this section only on root web.config file.