Is it possible to make time-slots that are already taken non-selectable in the back end?

When making an appointment in the back-end is it possible for the time slots that already taken to be non-selectable? Something like the calender on the front end of the website where it is impossible to book more than one appointment per time-slot. "Show scheduled users" add-on does show when appointments are, however it should be only for that time-slot that is selected.