I chose "never."

I have DID. Double-booking and then asking me to change schedules is extremely tough on me, given my condition. I would wonder if I dissociated and wasn't clear enough the last time we scheduled, or I would wonder why she bothered telling me she double-booked if I was the one who "won." If I was the one who "lost," I'd wonder if this would be a pattern, which would make my therapy counterproductive since the kind of therapy I need for my comorbidities comprise consistency from week to week (same day, same time every week). In my case right now, I see a T online through video twice a week - same days and same times every week. None of this weekly schedule changes crap. I can't handle that, and after many years of being frustrated and ghosting T's, I realized that I needed that boundary. So I asked for it, and some offered while others didn't. I chucked the ones who didn't and said, "Next!" It worked for me. Took some time, but it worked.

Everyone's different. Their needs are different. Some people can handle double-booking mistakes on an ongoing basis, whereas others just cannot.