On Windows XP I am working on a .NET 3.5 web app that's a combination of WebForms and MVC2 (The WebForms parts are legacy, and being migrated to MVC). When I run this from VS2008 using the ASP.NET web server everything works as expected.

However, when I host the app in IIS and try to use it, I see the following error

Section or group name 'cachingConfiguration' is already defined.
Updates to this may only occur at the configuration level where it is defined.

Source Error: 
Line 24:     </sectionGroup>
Line 25:     <section name="log4net" type="log4net.Config.Log4NetConfigurationSectionHandler, log4net"/>
Line 26:     <section name="cachingConfiguration" type="Microsoft.Practices.EnterpriseLibrary.Caching.Configuration.CacheManagerSettings,Microsoft.Practices.EnterpriseLibrary.Caching, Version=4.1.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" />
Line 27:   </configSections>
Line 28: 

Sure enough, if I remove the offending line (line 26 in the error message) from my web.config then the app runs correctly.

However, I really need to find out where the duplicate definition of this is. It's nowhere in my solution. Where else could it be?