Double reservation in the same room.

Hi there, 

Yesterday we had a couple booked into one of our rooms and for some reason a man was able to book the same room on booking.com for the same night later in the afternoon. our hotel was full last night, so we couldn't accommodate the extra booking and had to find alternative accommodation for him, he was very angry. Please can anyone shed a light on what has happened here? how has that been possible? we've also now lost the details on the room of the original booking, the couple that actually stayed in the room.