Clojure version of something josh was trying in Guava

multimap_thing.clj

(def input [["a" "apple"] ["a" "amazon"] ["b" "banana"] ["c" "clown"]])

(->> (group-by first input)              ; now we have our keys pointing to lists of every key value pair with that key
  (map (fn [[k v]] [k (map second v)]))  ; for each key value pair, keep only the second parts of the values
  (into {}))                             ; throw it into a hash map

;=> {"a" ("apple" "amazon"), "b" ("banana"), "c" ("clown")}