IIS7 deployment - duplicate 'system.web.extensions/scripting/scriptResourceHandler' section

Question

If your plan is to deploy to an IIS that has an Application Pool running in .net 4.0 you will need to cleanup the web.config that includes all the section Definitions that point to .net 3.5. The reason this fails is because these section definitions are already included in the root web.config in .NET 4.0 (see %windir%\microsoft.net\framework\v4.0.30319\config\machine.config) that include all the system.web.extensions declared already.

Another quick fix is to have the application pool set to 2.0 just as your development machine appears to have,.

The solution for me was to change the .NET framework version in the Application Pools from v4.0 to v2.0 for the Default App Pool:

If, like me, you need to target v4 but can only build with .net 3.5, follow the instruction here. Just replace in your web.config the whole content of the <configSections> with:

<configSections>
<sectionGroup name="system.web.extensions" type="System.Web.Configuration.SystemWebExtensionsSectionGroup, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
  <sectionGroup name="scripting" type="System.Web.Configuration.ScriptingSectionGroup, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
    <section name="scriptResourceHandler" type="System.Web.Configuration.ScriptingScriptResourceHandlerSection, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false" allowDefinition="MachineToApplication"/>
    <sectionGroup name="webServices" type="System.Web.Configuration.ScriptingWebServicesSectionGroup, System.Web.Extensions,  Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35">
      <section name="jsonSerialization" type="System.Web.Configuration.ScriptingJsonSerializationSection, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false" allowDefinition="Everywhere"/>
      <section name="profileService" type="System.Web.Configuration.ScriptingProfileServiceSection, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false" allowDefinition="MachineToApplication"/>
      <section name="authenticationService" type="System.Web.Configuration.ScriptingAuthenticationServiceSection, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false" allowDefinition="MachineToApplication"/>
      <section name="roleService" type="System.Web.Configuration.ScriptingRoleServiceSection, System.Web.Extensions, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" requirePermission="false" allowDefinition="MachineToApplication"/>
    </sectionGroup>
  </sectionGroup>
</sectionGroup>

Set the application pool to 2.0, I did it and worked.

Necromancing.
If you don't have any system.web.extensions config-sections or handler/module entries in your web.config, this problem is caused because you/somebody else copied a VisualStudio-Project (2013/2015/2017) while having hidden-files unhidden.

Because of that, it will not only copy .git, but also .VS, which contains an IIS-Express applicationhost.config file, which points to web.config files at paths that don't exist (or worse, paths that do exist, but do not have the same content)...

Solution:
Delete the applicationhost.config file in the .VS folder.
Or just delete the .VS folder altogether.
Visual Studio will re-create it.

My app was an ASP.Net3.5 app (using version 2 of the framework). When ASP.Net3.5 apps got created Visual Studio automatically added scriptResourceHandler to the web.config. Later versions of .Net put this into the machine.config. If you run your ASP.Net 3.5 app using the version 4 app pool (depending on install order this is the default app pool), you will get this error.

When I moved to using the version 2.0 app pool. The error went away. I then had to deal with the error when serving WCF .svc :

HTTP Error 404.17 - Not Found The requested content appears to be script and will not be served by the static file handler

After some investigation, it seems that I needed to register the WCF handler. using the following steps:

1. open Visual Studio Command Prompt (as administrator)
2. navigate to "C:\Windows\Microsoft.NET\Framework\v3.0\Windows Communication Foundation"
3. Run servicemodelreg -i

Apparently, other have (had) this problem. They rebuild in Framework 4.0. Can you?

Also, see this about making sure your IIS is fresh.