ASP.NET DISCO - Dynamic Discovery not working

Question

I'm trying to follow tutorial : https://flylib.com/books/en/2.439.1.34/1/

1. I created a 'Hello World' ASP.NET app (Web Forms template)
2. Added main.vsdisco file at root level it's content is minimal:

<dynamicDiscovery 
  xmlns="urn:schemas-dynamicdiscovery:disco.2000-03-17">
</dynamicDiscovery>

3. When running app in VS I got error: Request filtering is configured for the Web server and the file extension for this request is explicitly denied.
4. I added to the web.config:

<system.webServer>
    <security>
      <requestFiltering>
        <fileExtensions allowUnlisted="true">
          <add fileExtension=".vsdisco" allowed="true" />
        </fileExtensions>
      </requestFiltering>
    </security>
...

5. Now there is a startup error: Cannot add duplicate collection entry of type 'add' with unique key attribute 'fileExtension' set to '.vsdisco'

Can not see any duplication in the web.config