As per my recent experiences in international travel, "double" is a very vague term for the traveler and varies in meaning from place to place. The deciding factor tends to be the size of the room: is it big enough for two beds (singles, fulls, queens) or is it just one bed (full, queen or king) for up to two people. It's best to check with the hotel at the time of booking. A hotel that I chose not to stay in during a reunion in San Francisco recently, was going to be charged the rate for two people, even though I would have been using a queen bed all by myself (and that's why I chose a different hotel nearby).