By default, your cronjobs will automatically start at the scheduled time, even when there's another instance of it is running. It causes cronjob execution overlapping, which might bring undesired results when updating shared resources like databases.
You can now prevent your cronjob from overlapped executions by check the box Single Instance at the section Connect to your server when editing/adding cronjob.
|Enable Single Instance to prevent cronjob from overlapping.|
The feature is available for Platinum accounts or higher.