@Marjo8
Marjo, this is a known problem with the Airbnb booking system, and has been frustrating hosts for the past 4 years. Why they cannot fix it is one of the mysteries of the universe.
I will explain how this happens......
Under your Availability Settings is an item called 'Booking Window' and you have the ability to set how far ahead you are prepared to have guests book. You can set your personal default or, you can set 3, 6, 9, 12 months, or, 'All future dates'!
If you set your calendar to, lets say, 3 months........ prospective guests cannot book your listing any further ahead than 3 months from the current day. Now if you block dates that are 4 or 5 months in advance, they will show as blocked on your calendar but, as that rolling booking window comes to those dates you have blocked it will, (in some instances) open them up for booking and this is what happens!
You end up with a reservation for a date you had previously blocked.
As @Helen3 has said, get onto Airbnb and they will cancel the reservation as long as they can establish the reservation was for a blocked date.
God only knows why they do not fix this glitch, they have known about it for years but it seems anything that prevents a booking is not something that Airbnb encourages or wants to do anything about.
Marjo, the only way to guarantee this will not happen to you again is to set your booking window to 'All Future Dates' ! What this does is remove the rolling booking window from your calendar and you can then be confident any date you block into the future will stay blocked!
Cheers.....Rob