The duochrome in Bad Fairy reminded me of a Sally Hansen Nail Prism I'd gotten in recent swap, Bronze Ruby. I knew the Sally Hansen wasn't a dupe because it lacks the foil-y grittiness of the MAC, but I thought maybe layering might help with that. I first tried one coat of Sally Hansen Xtreme Wear Fireberry Red under two coats of Bronze Ruby on my middle finger. That combination was no Bad Fairy, though in the sun it did look sort of similar.
Next I tried two coats of Orly Shine (a silver foil) under two coats of Bronze Ruby to try and get that metallic-y thing going. The texture of that combination was closer to Bad Fairy than my first try, but the color was still off. I did like the effect of the Prism over the foil, so I plan to do some more experiments along those lines to see what I can come up with.
I've seen mentions of other polishes being dupe-y to Bad Fairy. OPI DS Perfection is one leading candidate (not that it's easy to find anymore), though All Things Nails found it to be not that close. Same story with OPI The Show Must Go On according to The Lacquerista. Milani Hot Flash from this past summer has a similar texture and base color but lacks the duochrome. Accessorize Pink Spice seems to be the current front runner in the dupe race, but some say it's not a perfect match either. Right now, I'm feeling glad I was able to get Bad Fairy.