@Paul-And-Sharon2 When I first joined Airbnb I had exactly the same problem, and it was at that stage that I discovered that it's not so much who booked first, so much as being with Airbnb means that as far as they're concerned, a booking on their platform takes precedence over any other booking for the same dates.
That was a very steep learning curve, but once resolved (I had to cancel the other booking, even though it had come through seconds before Airbnb's own booking) that turned out to be the first and last occasion we ever had such a double booking!
@Pat271 has all the right things to do, whilst @Tim-and-Holly0 kindly highlighted what can happen on other platforms/calendars.
@Paul-And-Sharon2 I hope you manage to resolve the situation without too much upset.