basically folder admin + pihole are just symbolic links into folder /var/www/html. There are not duplicates. This is needed due to the way PiHole works.
if you require an additional web server application, you would create them into a separat sub folder like /var/www/app1 or /var/www/app2. On your browser you could simply open these web sides using http://your.ip/app1 or http://your.ip/app2