Double dipping is fun, but you don't gain any damage. FL gets clipped when you double cast, so you only get 6 tics of damage in 3 seconds. If you fast cast FL just once, you get the normal 4x tics in 1.5 seconds, and then cast a normal 4x tic 3 second cast gives the same damage. I still double dip, but it is good to know you don't lose anything by missing the second cast.