|These guys know all about doubles.|
Here's where the counterintuitive part comes in--adding a second run can increase your recovery! [Note: Now would be a great time to revisit my post on recovery runs if you haven't read it]. For starters, a long run is tougher on your body than two short runs even if the total distance is the same. Secondly, if you do doubles with some regularity, it stimulates your body to adapt--you're getting an aerobic stimulus two times in one day!
|My favorite kind of double.|
There's another obvious plus to doubles. In periods of extreme weather, like 90+ degree summer days or frigid winter days, it can be much easier psychologically and physiologically to spend two short periods outside rather than a single extended period. Yes there's much to be said for acclimating yourself to the kinds of conditions you may face on race day, but on easy days, doubles can help you avoid the extra stress that extreme temperatures place on the body.
One word of caution, be careful adding doubles to your schedule. I recommend this great article from Running Times, which details a smart approach to incorporating doubles.