Double Booking Dark Pattern on AirBnB

I recently booked a single night stay in London with AirBnB, who promptly told me on completion of payment that I already had a booking confirmed for that night, and would I like to cancel one of them?

It’s easy to cancel, but you don’t get a full refund; AirBnB keep the booking fee, which was about £10 in my case. The obvious question here is that if AirBnB knew I’d already booked a place for that date and even told me after booking, why didn’t they tell me before payment confirmation?

The answer is obvious. Sometimes there are shades of grey when it comes to business, but this is wholly wrong.