Double booking for 1 room when calendar shows that is already booked and also minimum stays

We have recently registered with Booking.com and have a booking for next weekend - however another booking has come through today for the same dates when our calendar clearly shows the earlier booking.

We understand that we are now liable to pay for alternative accommodation - how can this be when the calendar clearly showed that our 1 room was booked out?  

We have also requested an amendment to our address and requested that we will only accept a minimum of 2 night stays.  I understand that this amendment could take up to 4 weeks!!

Why is there not a chat line for Hosts as there is for guests?

Sorry to rant but so frustrated and thinking of terminating the contract after our first booking as we may well be out of pocket anyway if having to pay for alternative accommodation for the double booking and of course we will have to wait for payment of our first guest so perhaps we wont cancel until after we have had that!!!