It's there. Look to the left of the main rainbow. Yesterday could only be described as a monsoon. Torrents of rain. Then late-afternoon sun. Then rain with the sun for a couple of hours. Hence: double rainbow.
Tangential thought: there used to be dozens of Double Rainbow ice cream stores all over San Francisco and the Bay Area. Now Double Rainbow ice cream is only available in a limited number of grocery stores. The ice cream has become as elusive as its name.