Unfortunate that your therapist double booked, but it happens at times with therapist and with other professionals. I have seen three therapist in the last six years and every single one of them over booked me at one point and usually they were exceptionally good about things like that. Though it feels bad, twice in five years is not a bad record. It can be as simple as looking at the wrong week or month or not focusing on what they are doing at that moment.

When it happened to me it was based on who made the appointment first is how they did it. I chose in two instances to allow the other person to have it and rebook.