There's pretty much zero chance of a double exposure---the film came out of its packaging, went in the camera, went straight through in a day, and got an identifying label stuck in the canister with it when it came out. (And I saw no rainbows that day; it was uniformly grey from dawn to dusk.)

So while it may be possible for the image of a "real" rainbow to look like that, I'm willing to exclude that possibility from consideration.