I just figured it out. Here's how I do it:

[VBA]Private Sub List1AddButton_Click()
Dim i As Variant
If ListBox1.ListIndex = -1 Then Exit Sub
For i = 0 To ListBox2.ListCount - 1
If ListBox1.Value = List2.List(i) Then
Exit Sub
End If
Next i
ListBox2.AddItem ListBox1.List(i)
End Sub[/VBA]