we e arrived at the apartment at about 16:40 to find out it's not vacant. they accidently double booked it.... happens ..
the real problem was the handling that followed the issue. Though being very nice, the company was not really responsive and understanding. they did try to solve the situation but in a very slow manner (me and my wife with kids after rough trip to porto + luggage). they did find a vacant property which they own 8 minutes away but it took 1.5 hours to provide us with check-in details and an address (self check in). have to say that both the occupied and the vacant property were really nice.