ASP.Net Duplicate Section Defined Error - Is Only Defined Once

I had just set up a new local development environment for a solution already up and running in a staging environment.

The first time I tried to run the app, I got this error message:

Config Error There is a duplicate 'system.web.extensions/scripting/scriptResourceHandler' section

Here are the exception details:

HTTP Error 500.19 - Internal Server Error
The requested page cannot be accessed because the related configuration data for the page is invalid.
Detailed Error Information
Module IIS Web Core
Notification Unknown
Handler Not yet determined
Error Code 0x800700b7
Config Error There is a duplicate 'system.web.extensions/scripting/scriptResourceHandler' section defined
Config File \\?\C:\SVN\TestSite\web.config

...but if you look in the web.config, the scriptResourceHandler is only defined once. However, if you're using .Net Framework 4.0, this entry already exists in the machine.config - hence the error. So the solution is, either remove this and other duplicate entries from your web config, or if it's an option, change your site in IIS from using the Framework 4.0 application pool back to the Framework 2.0 application pool.