A double rainbow is not as uncommon as you might think. The secondary rainbow is the result of sunlight that has undergone two internal reflections (rather than one, for the primary) in the water droplets you're viewing (with your back to the sun). One of the signature features of the secondary rainbow is the reversal of colors relative to the primary, with red on the inside and violet on the outside. Here's a video that explains these phenomena rather well in under four minutes: https://www.youtube.com/watch?v=xkDhQGXqwCM
On Kaanapali Beach in West Maui, we usually see rainbows late in the day; the setting sun illuminates rain clouds clinging to the West Maui mountains. In the case pictured here, though, I'm looking west at about 8 am; the sun has risen above the mountains behind me and is shining down on a little water vapor over the ocean. With the rainbow arches lower in the sky, it's easier to see that faint second rainbow.
For my photographer friends, here are a few technical notes. I shot the image with a full-frame mirrorless camera, the Sony a7R2, with a Zeiss 24-70 mm lens set at 24 mm. Exposure was f/10 @ 1/640 sec at ISO 320. In post-processing, I applied a Nik polarizing filter, which has the effect of darkening the sky, reducing reflections from the ocean surface, and increasing saturation and contrast in the rainbows.