Cron job runs on both master and mirror server, why?

[snowfly]

I have 2 web servers running ISPconfig 3, first server is master, and second is set to be "Is mirror of Server" master.

When creating cron jobs on the master server using ISPConfig, it appears these cron jobs are replicated across on the mirror.
Hence our crons are run twice.

e.g. I have a cron to hit a PHP script (URL cron), and the PHP script sends me an email.
I get the email twice, because both the master and mirror server run the cron.

How do I get the mirror server to not run crons?
I only use the mirror server as a backup, and ISPConfig mirrors all the config, and I use rsync to mirror data.