**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:

>!  http://atoptics.co.uk/rainbows/bowim6.htm
>!  <br><br>
>!  <img src="https://i.sstatic.net/qGxkn.png">

And @justhalf found another image which looks even more like the one in the question, except rotated 90 degrees:

>!  <img src="https://i.sstatic.net/BK9L5.png">
>!  <br>
>!  [Atmospheric Optics](http://atoptics.co.uk/rainbows/bowim47.htm)
<hr>

I'm thinking that the **actual answer** is something to do with

>!  light **reflecting** off the **windows** of a **high-rise building**

<br> <hr>

**Added** layout from puzzle&rsquo;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&rsquo;s reflection,
>! as if there were a second sun,
>! but neither rainbow&rsquo;s right half is illuminated
>! for different reasons.
>! <br><sup><sup></sup></sup> &bull;
>!     A portion of sunlight for the directly-lit rainbow
>!     is blocked by the building.
>! <br><sup><sup></sup></sup> &bull;
>!     Reflected sunlight that would complete
>!     the second rainbow (dimmer and left shifted)
>!     is not present due to
>!     the viewpoint&rsquo;s position
>!     and the sun&rsquo;s angle.
>! <br><sup><sup></sup></sup>
>! The building&rsquo;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&deg; differently.
>! <br><br>
>! <img src="https://i.sstatic.net/waPhF.png">

For comparison, shadows are often doubled in a related manner:

<img src="https://i.sstatic.net/ROSxA.png">