I'm always a fan of sharing with you all a little of my Jamaican heritage. The word "Duppy" means ghost. There are quite a few "Duppy Stories" in Jamaica, and often told to scare the shit out of children! lol.
So when someone says, "Duppy know who fi Frighten!", think about the Duppy as the person who's trying to get under your skin. They're the duppy! It means, bully's know who. tomess with, and who not to mess with!
Don't make that Duppy Frighten you!