If you try to create Appointments that has already another Appointment existed with the same time slot,
you will see the Scheduling Alerts which you can still ignore and save them.
Also, if you open the corresponding Appointment, Scheduling Alert is displayed on the top of the screen.

It would be great that if we can set the Scheduling Alert to either turn ON/OFF for Appointment entity,

Version : CRM Online 2015, 2016