pack1.clj

(defn pack [coll]
  "pack duplicates into sublists"
  (loop [acc '(())
         fst (first coll)
         rst (rest coll)]
    (let [current (first (first acc))
          added-to-current (cons (cons fst (first acc)) (rest acc))
          added-new-sublist (cons (list fst) acc)]
      (if (seq rst)
        (if (or (nil? current) (= current fst))
          (recur added-to-current (first rst) (rest rst))
          (recur added-new-sublist (first rst) (rest rst)))
        (reverse (if (= current fst)
                    added-to-current
                    added-new-sublist))))))

pack2.clj

(defn pack [coll]
  (partition-by identity coll))