I bought a physical copy of Civilization V: Game of the Year Edition, and the game arrived, a day later. I installed the game, and entered the product code through Steam. It said the product code had already been used, but I had purchased it brand new.
I thought that somebody may have used the wrong key, and that it could be an accident, so I sent the game back for a replacement. Nevertheless, I had the same issue with the new product key. It is too unlikely that it is a coincidence, having happened twice, in a row.
So I wonder why it doesn't work. I haven't made a mistake entering the code, because I checked it several times. The site where I bought the game is a trusted site. What else could be causing this?