The first appointment filter is applied per report. This means that if a reminder report has the same patient multiple times, a text will only be sent for the earliest appointment. If your office has 1 hour before reminders set up for patients, this means that multiple reports are being sent for your office every day. Because there are multiple reports throughout the day, this can cause a patient to receive multiple reminders if their appointments are scheduled at different times.
Let's say I have a Chiro appointment with your office at 8:30 am and then a Regen Med appointment at 8:45 am. As a result of your reminders are sent 1 hour before each specific appointment, the 8:30 am reminder will be on a report sent at around 7:30 am and the 8:45 am reminder will be on a different report sent around 7:45 am. Since the appointments were scheduled at different times and the reminders are set to send exactly 1 hour from the appointment, my 2 appointments will appear on 2 different reports.
The solution for this would be to set reminders to send all at once the morning of the appointment instead of exactly 1 hour before the appointment. This will ensure that all of the appointments are sent in one report (since they are all sent at one set time) and the first appointment filter will only send the earliest reminder on the report.