say hello https://groups.google.com/forum/#!topic/clojure/g09Jb-7kKnk

say_hello.clj

(ns say-hello)

(defn say-hello
  ([names] (say-hello names #{}))
  ([names saluted]
     (reduce (fn [saluted name]
               (if-not (saluted name)
                 (do (println "Hello" name "!")
                     (conj saluted name))
                 (do (println "Welcome Back" name "!")
                     saluted)))
             saluted names)
     (println "Goodbye!")))

;; (say-hello  ["Peter" "Pablo" "John" "Peter"])
;; =>
;; Hello Peter !
;; Hello Pablo !
;; Hello John !
;; Welcome Back Peter !
;; Goodbye!