Cron set in Drupal UI or by server?
Drupal has a built in "poor man's cron" which is invoked on a schedule when regular users visit pages.

Setting up a proper crontab entry (and turning off poor man's cron, i.e. set it to "Never") is usually the best thing to do.

If you set both, the poor man's (in-built) cron and also add a crontab entry to ping the cron URL, then yes the cron will run twice.