Preventing double booking is set at the User level. Doing so only prevents double booking for you. Each Staff Member will need to go in and update their own settings.
Turn Off Double Booking
NOTE: Use the Can modify setting to allow multiple appointments in same slot permission in Default Permissions to control who can edit this setting. Learn more.
- Click your Name in top right corner and select Calendar Preferences.
- Toggle No for Allow booking multiple appointments in same time slot.