HTTP Error 500.19 - Internal Server Error” with Dynamic Data

I have a simple Dynamic Data application set up using forms authentication as the admin section of another web application. This works when I run it inside Visual Studio as a standalone project.

I've deployed it to my live site inside an application directory and I get the following error:

HTTP Error 500.19 - Internal Server Error The requested page cannot be accessed because the related configuration data for the page is invalid.

Module IIS Web Core
Notification BeginRequest
Handler Not yet determined
Error Code 0x800700b7
Config Error Cannot add duplicate collection entry of type 'add' with unique key attribute 'name' set to 'UrlRoutingHandler'
Config File \?\C:\sites\website1\dd\web.config

Config source:

<add name="ScriptResource" preCondition="integratedMode" verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35"/>
<add name="UrlRoutingHandler" preCondition="integratedMode" verb="*" path="UrlRouting.axd" type="System.Web.HttpForbiddenHandler, System.Web, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"/>
</handlers>

This error message normally indicates a "bitness" problem.

Are you running a 64 bit dll from a 32 bit application pool?

This would match that you only get the problem when you run it in the IIS app pool and not in VS.