Соединение двух списков без повторения

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim list3 As New ArrayList
 
        For Each List1 As Integer In ListBox1.Items
            Dim iFound As Boolean = False
            For Each lst As Integer In list3
                If List1 = lst Then
                    iFound = True
                    Exit For
                End If
            Next
            If Not iFound Then list3.Add(List1)
        Next
 
        For Each List2 As Integer In ListBox2.Items
            Dim iFound As Boolean = False
            For Each lst As Integer In list3
                If List2 = lst Then
                    iFound = True
                    Exit For
                End If
            Next
            If Not iFound Then list3.Add(List2)
        Next
 
        list3.Sort()
 
        ListBox3.Items.AddRange(list3.ToArray)
    End Sub

        Dim M1() As Integer = {100, 25, 300, 400, 12000}
        Dim M2() As Integer = {10, 25, 25, 500}
        Dim Result As New List(Of Integer)
        Result.AddRange(M1)
        Result.AddRange(M2.Except(M1).ToArray)
        Result.Sort()
        ListBox1.Items.AddRange(Result.[Select](Function(x) x.ToString()).ToArray())

Dim li1 As New List(Of Integer)
Dim li2 As New List(Of Integer)
li1.AddRange({100, 25, 300, 400, 12000})
li2.AddRange({10, 25, 25, 500})
li1.AddRange(li2)
Dim li0 = Enumerable.Distinct(li1).ToArray
Array.Sort(li0)
ListBox1.DataSource = li0