@Hasita0Was the dates that the other guest was able to book at the begging or end of your blocked period, or was it in the middle? Remember that if you block a date for booking, guests can still check out on that date.
However, if you get an instant booking for dates that should have been blocked in your calendar, just call Airbnb and have them cancel the booking penalty-free. You can report the issue of the dates not being blocked at the same time.