What Causes a Double Rainbow?
When looking at a normal rainbow, what you're viewing is a primary rainbow. But sometimes a secondary rainbow will occur, and this is referred to as a double rainbow. This secondary rainbow arises from two internal reflections and the rays exit the drop at an angle of 50 degrees rather than the 42 degrees for the primary rainbow. The order of the colors of the secondary rainbow is reverse that of the primary. The reds are on the inside and the blues are on the outside of the colored arc.
http://www.cs.princeton.edu/~rywang/berkeley/magic_small/rainbows.html