Bookings Cancelling in Error

We have slots set up for 3 unique customers to book into one time slot.  Notifications are set up to send a confirmation and Outlook invitation.  When the first person books their time slot, everything works perfectly and they receive the confirmation and Outlook invitation.  When the second customer books the same time slot, they receive the confirmation and an Outlook invitation but the first customer receives an Outlook cancellation.  The same occurs with the third customer, the second customer receives the Outlook cancellation.

All of the customers are still on the Bookings calendar.