Double rainbows aren't all that rare ... they centre around the point in the sky opposite the sun, so unless you're in a plane or a balloon, the best time to look for them is when the sun is low in the sky, and rain is in the opposite direction.
Another refractive effect to look for is the halo, which occurs around the sun or the moon (as a rather faint sort of rainbow). Maybe 30 degrees in diameter? It indicates the presence of high ice-crystals, cirrus cloud, which in temperate latitudes, is generally a precursor to a change in the weather. It's often there, but no-one in the busy streets below bothers to look for it.
There's an old English saying (at least according to my "Cloud Study" manual) "when the moon is in it's house, (the halo) it will rain". Akin to "red sky in the morning, shepherd's warning, red sky at night, shepherd's delight".
I've tried this out on the shepherds here, but they were too busy roasting their jumbucks.
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