Regular rainbows occur when moisture in the air refracts sunlight in such a way that it is broken up into its constituent colours.
The phenomenon occurs when the Sun is positioned behind you and sunlight passes through the airborne water. The light refracts (bends) inside the droplets and the white light is broken up. Each colour has a different wavelength so, depending on the angle of refraction, a different colour of light will be reflected outwards, the result of this process is what we observe when we see a rainbow.
Every rainbow is accompanied by another, secondary rainbow. but it’s usually too dim to see. This double rainbow effect is due to the continued reflection of light inside each water drop.
Sunlight is actually reflected twice inside a drop: once to produce the primary rainbow and a second time at the back of the drop. This second reflection inverts the light but undergoes the same refraction, so exits in the same way as before – though upside down.
This second reflection reduces the intensity of the sunlight, but it also produces a second inverted rainbow, creating a double arc of multicoloured light.