How can I avoid double booking?

Ensure that you keep the dates you are unavailable for any tests at all up to date in your instructor account details. We will ignore any tests that become available within those date ranges. This is honored for all searches where your ADI number has been provided whether or not the account is a Learner Account, one of your Pupil allowance or your subscription payments are up to date.
Edit your active Pupil and Learner accounts to fine tune available dates ie the last afternoon of an intensive course might only be acceptable for the pupil on that course.
Register as a business with the DVSA and ensure that your ADI number is included in all test bookings. The DVSA will then ensure, within some limitations, that you are not booked for two at the same time or too close together. For more information see https://www.gov.uk/manage-approved-driving-instructor-registration/change-when-youre-available-for-practical-tests