duplicateCheck.cs

private void handleFilterDuplicate() {
    this.lists.Clear();
    listAll.Items.Clear();

    for (int i = 0; i < list1.Items.Count; i++ )
    {
        string a = list1.Items[i].ToString();
        Console.WriteLine(a);
        if (isNumExist(Int32.Parse(a)) == false)
        {
            this.lists.Add(Int32.Parse(a));
        }
    }

    for (int i = 0; i < list2.Items.Count; i++)
    {
        string a = list2.Items[i].ToString();
        if (isNumExist(Int32.Parse(a)) == false)
        {
            this.lists.Add(Int32.Parse(a));
        }
    }

    this.lists.ForEach(item =>
    {
        listAll.Items.Add(item);

    });
}