foobar.clj

(ns foobar
  (:require [clojure.test :refer [deftest is testing]]))

(defn init-count [hm k]
  (assoc hm k 0))

(defn inc-count [hm k]
  (update-in hm [k] inc))

(defn find-dup [coll]
  (let [counts (reduce init-count {} (set coll))
        max-entry (apply max-key val (reduce inc-count counts coll))]
    (when (> (val max-entry) 1) (key max-entry))))

(deftest find-dup-tests
  (testing "finds first occurency of adjacent repeated values"
    (is (= 3 (find-dup [1 2 3 3 4 5]))))

  (testing "find nothing when there are no repeated values"
    (is (nil? (find-dup [1 2 3 4 5]))))

  (testing "finds first ocurrency of non-adjacent repeated values"
    (is (= 3 (find-dup [1 2 3 4 5 3]))))

  (testing "finds most frequent repeated values"
    (is (= 2 (find-dup [1 2 3 4 5 3 2 2 4])))))

There is a specific scenario in which there can be only one duplicate number in the collection. In that case, finding the duplicate is as simple as finding the collection intersection with its set:

(defn find-only-dup [ints]
  (- (reduce + ints) (reduce + (set ints))))