Could a technical issue send two identical confirmations?+
No. The system claims the send job on the booking record before anything goes out. If the job runs again for any reason, it sees the work is already done and skips it.
Do SMS and email always mirror each other?+
Not necessarily. You control each channel independently. You can run email only, SMS only, or both, depending on your preference and what each client has agreed to receive.
Is this the same as marketing messages?+
No. Appointment confirmations, reminders, and aftercare are separate from marketing campaigns. Clients receive these messages based on their bookings, regardless of whether they have ever opted into a promotional list.