Double flats are not that common but failure to properly diagnose and/or repair a flat can be resulting in the need for 2nd repair immediately. Like most others I bring both tubes and a patch kit.
(If you get an obvious location of a simple puncture (say a thorn stuck on outside of tire) it can often be quicker to keep wheel on bike, pull out the affected section of tube, patch it, make sure thorn is fully gone, stuff it back, inflate and go than a full tube replacement. I learned this from my fixie days when I didn't like to remove my rear wheel.)