Implementation of map-invert that preserves duplicates by cons entries into a sequence.

map-invert-preserve-dups.clj

;; See: http://codereview.stackexchange.com/q/6682/9032
(defn map-invert-preserve-dups [m]
  (apply merge-with into
         (for [[k v] m]
           {v [k]})))

(->> "http://www.weeklyscript.com/Pulp%20Fiction.txt" 
   (slurp)
   (re-seq #"\w{5,}")
   (frequencies)
   (map-invert-preserve-dups)
   (sort)
   (reverse)
   (take 25))

([272 ("VINCENT")] [214 ("JULES")] [182 ("Butch")] [162 ("Vincent")] [147 ("BUTCH")] [94 ("Jules")] [92 ("FABIENNE")] [89 ("fuckin")] [78 ("Marsellus")] [77 ("gonna")] [58 ("LANCE")] [56 ("about")] [54 ("YOUNG")] [51 ("through")] [50 ("looks" "there")] [45 ("never")] [44 ("front")] [43 ("Jimmie" "little")] [42 ("their")] [41 ("right" "NIGHT" "takes")] [37 ("JIMMIE" "other")] [35 ("still")] [34 ("PUMPKIN")] [33 ("Fabienne")] [32 ("think" "watch" "table" "Maynard")])