calendar synchronicity

I received two reservations at the same time and for the same apartment (one from booking, one from air bnb) and both were confirmed by both providers. I have two guests for the same apartment coming this Saturday - one from booking and one from air bnb - I am quite nervous about this. How did this happen? The calendars are synchronized ... pls help me

What should I do? None of them wants to cancel the reservation of course ...