I have not been keeping log files for my web server until the beginning of 2016, when I needed to trace access to certain files (I started to use awstats, mostly because I was familiar with it since I had used it years ago when my site was still running on a Red Hat server). In March 2016 I luckily started to use Let's Encrypt. I had used another "free" service before, which got recently into big trouble as they apparently had not control over their own security. When I looked at my Apache server's log files (on Ubuntu 14.04), I noticed that apache did double logging (to both the individual vhost's log file and a common log file). I realized that Let's Encrypt specifies into every vhost's configuration file an Import directive which sources /etc/letsencrypt/options-ssl-apache.conf. And this file in turn specifies common access.log and error.log files for all vhosts in the /var/log/apache2/ directory. I uncommented the five lines associated with this logging and the duplicate logging stopped (originally, I had thought, that this letsencrypt directive was only used for the initial Let's Encrypt setup for the cert generation).