Allow only one appointment to be booked per email address

Please advise, I would like to block customers from booking multiple slots. So only one appointment is allowed.

For example,

a customer A book an appointment on 25 June 2019, and also went ahead to book 2 July 2019 and 3 July 2019.

So if an appointment is already booked on a certain date, I would like to prevent a customer from booking another appointment.