firstDuplicate -TizzyT

        static int firstDuplicate(int[] a) {
            object[] values = new object[a.Length];
            for (int i = 0, idx; i < a.Length; i++)
                if (values[idx = a[i] - 1] == null) values[idx] = new object();
                else return ++idx;
            return -1;
        }