clj.clj

(defn- copy-nil-vals [[k v]]
  (if (nil? v)
    {k k}
    {k v}))

(defn column-vals [column-mappings]
  (->> column-mappings
    (map #(select-keys % [:value :mapped_value]))
    (map vals)
    (map #(apply hash-map %))
    (apply merge)
    (map copy-nil-vals)
    (apply merge)))

(column-vals [{:value "h1" :mapped_value "header1" :value "h2" :mapped_value nil}])

should return

{"h1" "header1" "h2" "h2"}