Hoy is a slang term used to politely remind an individual that they are repeating a story that they have already told you. The root of this stems from a conversation someone had with an acquaintance shortly after having a medical procedure involving conscious sedation using an injectible amnesiac (midazolam). This individual kept repeating the same story over and over the key word of the story was about project of "hoy" construction. Without recollection this individual had mentioned the story 5 times on the car-ride home.
Whenever someone begins to tell you a story for the second time you can politely say "hoy" to remind them you have heard it before.