Brewing coffee with coffee will result in a stronger-tasting coffee. Coffee aficionados call it a "double brew." And, since you’re either doubling the amount of ground coffee or the amount of coffee altogether, you'll get double the amount of caffeine you'd normally get in a standard coffee. Unlike milk, running coffee through a coffee maker won't destroy your machine. Regardless, it'll make your coffee taste extremely bitter. So, if you like your coffee bitter, go for it!