Original solution by YowE3K (who later turned it into this community wiki)
This isn't an answer to the exact question, but the following link is to an image that I thought was worth looking at anyway:
And @justhalf found another image which looks even more like the one in the question, except rotated 90 degrees:
I'm thinking that the actual answer is something to do with
light reflecting off the windows of a high-rise building
Added layout from puzzle’s poser
The rightmost diagram represents what can happen when the sun sets in a direction almost parallel to the wall of a high-rise building (whose corner is the gray rectangular area with writing) and reflects off its windows. A second rainbow is created by the sun’s reflection, as if there were a second sun, but neither rainbow’s right half is illuminated for different reasons.
• A portion of sunlight for the directly-lit rainbow is blocked by the building.
• Reflected sunlight that would complete the second rainbow (dimmer and left shifted) is not present due to the viewpoint’s position and the sun’s angle.
The building’s vertical wall plays the same reflective role as horizontal bodies of water in the photographs above, which is why the doubling is oriented 90° differently.
For comparison, shadows are often doubled in a related manner:
