Yes!! Sometimes we have seen that calendars accept the events automatically even it was booked before, So not to worry much we can avoid double booking situation with our resources.
Avoid Double Booking
Go to your calendar page.
Add resource calendar to your calendar.
Go to calendar settings for that particular resource and then you can see the “Automatically accept all invitations to this calendar” option has been set by default under Auto-accept invitations. Update that setting to “Auto accept invitations that do not conflict”
Now this calendar will refuse all events invitations on time slot where it is already booked.