Public Class int FirstDuplicate(int[] a) {

    int count = a.Length;

    for (int i = 0; i < count-1;i++){

        for (int j = 1; j <count;j++){

        if (a[i] == a[j])

            return a[j];

        }

    }

    return -1;

}

​

//FirstDuplicate (1,2,3,4,5);