I have two ListBoxes. ListBox1 has list of items that can be selected by the user to transfer to ListBox2 by either double clicking the item or pressing the add button. What I want to do now is to prevent the user from adding duplicates in ListBox2. If ever a duplicate is detected a message will prompt "Item already included" and end the code. I am guessing this can be done with contains? But I have no idea how to do it. I have the following codes:

'Report Listing
Private Sub UserForm_Initialize()
    'List of Reports
    With ListBox1
        .AddItem "Report 1"
        .AddItem "Report 2"
        .AddItem "Report 3"
        .AddItem "Report 4"
        .AddItem "Report 5"
        .AddItem "Report 6"
    End With
End Sub

'Add selection to ListBox2
Private Sub AddButton_Click()

        With ListBox1
        Dim itemIndex As Integer
        For itemIndex = .ListCount - 1 To 0 Step -1
            If .Selected(itemIndex) Then
                ListBox2.AddItem .List(itemIndex)
            End If
        Next itemIndex
    End With

End Sub

    'Double click to Add
Private Sub ListBox1_DblClick(ByVal Cancel As MSForms.ReturnBoolean)
    ListBox2.AddItem ListBox1.List(ListBox1.ListIndex)
End Sub