The most naive way of solving this problem by a person who is super new to Closure, but I did get joy from solving this nonetheless :)

first-reccurring-character-problem.clj

(defn first-reccurring-character
  ([string-sequence] (first-reccurring-character string-sequence #{}))
  ([string-sequence lookup]
   (let [current-character (first string-sequence)]
     (if (contains? lookup current-character)
       current-character
       (recur (rest string-sequence) (conj lookup current-character))))))