Failed to encrypt the section [name] using provider RsaProtectedConfigurationProvider. Error message from the provider: Object already exists.

If you're running aspnet_regiis to encrypt a section in your web.config, e.g.

 aspnet_regiis -pef mytestsection .

(assuming your current directory contains the web.config and the section is called 'mytestsection')

And you get a message ...   Object already exists  trying running the Visual Studio command prompt as an administrator.