As others have said, this sounds like an authentic mistake- and the photographer's communication and apology are good signs. 2021 and 22 were/are going to be absolutely crazy for the industry.
2 months ago, I nearly double-booked a wedding date, but luckily caught things right before I sent a contract to my second clients. I was so embarrassed, and offered my other client a discounted portrait session. That was the first time anything like that has happened to me. This was a huge wake-up call, even after using all my calendars and apps- this almost slipped through.
Bear with us as we all navigate the next few years. I would say give this photographer another chance, but that is of course up to your comfort level :) Congratulations, by the way!