The double rainbow thing is kind of old right now, but old is new, right? That's what 343 seems to think, and either way, it likely didn't take a ton of effort to copy and paste this rainbow an extra time on the level "Reclaimer."
After the Mammoth crosses a river next to a waterfall, if you look around carefully you'll be able to see the double rainbow in all its dual glory. DOUBLE RAINBOW ALL THE WAY ACROSS THE SKYYYY! Ahem.
[via CVG, Gamerant]