Every user has control over their preferences which allow double booking or not.
Before You Begin
These settings are your personal settings. If you're an admin and you disable double booking - you're only disabling it for you. You must have each staff member go in and update this setting.
Turn Off Double Booking
- Click on your name in top-right corner of the global navigation bar
- Click Calendar Preferences from the dropdown menu
- Scroll down to calendar preferences below general preference
- To prevent double booking of appointments on your calendar switch the "Allow booking multiple appointments appointments in same time slot" to "No".