If your cron execute more than once every day, each time, the plugin will send a reminder.
With this :
the plugin will check the last executed date of every reminder with the current date and won't send a new email if the reminder was already send for the date.
instead of updating the executed_at for each role for each mail (and don't if there is no one), the rake action will save the executed_at every time (and only once) the reminder is executed.
If your cron execute more than once every day, each time, the plugin will send a reminder. With this :