This is annoying error which occurs when you try to start, restart or
check apache configuration.
This error is caused by duplicate NameVirtualHost entries. In my case in Debian.
I had to comment it in /etc/apache2/ports.conf . Apparently,
was mentioned twice in my overall Apache configuration files.
I should note that this is just a warning so Apache will be working just fine, even when the error appears on Apache start-up.