After shipping your asp.net 2.0, 3.0 or 3.5 application to a new server you get the following error:
There is a duplicate ‘system.web.extensions/scripting/scriptResourceHandler’
Don’t worry you haven’t done anything wrong, your are just trying to ship to a new server that has been configured to use .NET 4.0 as the default, and lucky for you it is a quick and simple configuration change to fix it:
Change Application Pool to .Net Framework v2.0
Open Internet Information Services (IIS) Manager
Goto the Application Pools screen
Open the Application Pool offending web application(s) are using
Change the Application Pool to use dotnet 2.0 instead of .net 4.0
Hard Refresh the erroring page with a <Ctrl> F5