I fought with this error as well. My solution was similar, but a bit different, to Adam's solution.
In my case, I had Sitefinity installed to: C:\inetpub\wwwroot
Then I had another Sitefinity web site installed to: C:\inetpub\wwwroot\SitefinityCommunityEdition
The 2nd Sitefinity web site existed as its own application in IIS, but I was still getting interference between these 2 Sitefinity installations. When I removed the Sitefinity installation from C:\inetpub\wwwroot the error (and duplicated PageIndex provider) disappeared.
I also noticed the error can be corrected by giving the PageIndexer a new name (PageIndex2) in the 2nd Sitefinity web site's web.config:
| <add name="PageIndex2" type="Telerik.Cms.Search.PageIndexProvider, Telerik.Cms" settingsControl="Telerik.Cms.Web.UI.PageIndexSettings, Telerik.Cms" viewSettingsControl="Telerik.Cms.Web.UI.SearchViewControl, Telerik.Cms" description="Provides indexing services for CMS Pages." />
This is not a great solution though. It's better to setup an environment where the two Sitefinity web sites aren't interferring with each other.