Double runs are simply a natural extension of increased tolerance to training stimulus. All you are doing is decreasing the recovery time between runs. A beginning runner may only run every other day. Eventually, she will progress to running 4, 5 and then 6 days a week. Some would advocate taking a day off every week, but a week is a fairly arbitrary unit for a training cycle. There’s no reason why a runner shouldn’t continue to run for 7, 8, 10 consecutive days. Certainly listening to the body is important, but in order to get better, you have to progress. So at a certain point, you are going to want to consider doubles. A morning run, usually shorter and easy, followed by another easier run in the evening. The next progression would be to run easy in the morning, and do a workout later in the day. The best runners in the world are doing some quality running every day, and are hitting three sessions per day. There’s always a next level to progress to.
The Summer of Malmo is a bit of a cult classic that describes how to build up your fitness over the summer (or winter, depending on your plan) with doubles as a significant aspect of the training.