How to fix "The entry 'ScriptModule' has already been added." error

The Problem: The web application works just fine on the local machine, but when you upload it to the web server you get the error "The entry 'ScriptModule' has already been added."

The Cause: It is a conflict between asp.net versions.

The Solution: Simply comment out the <add name="ScriptModule" type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/> line in the web.config file and you should be good to go.