Private Sub List2_DblClick()
  Dim bFound As Boolean
  Dim i As Integer
 
  With Form1.List1
    For i = 0 To .ListCount - 1
      If .List(i) = List2.Text Then
        bFound = True: Exit For
      End If
    Next i
 
    If Not bFound Then .AddItem List2.Text
  End With
End Sub

Private Declare Function SendMessage Lib "user32" Alias _
        "SendMessageA" (ByVal hwnd As Long, ByVal wMsg _
        As Long, ByVal wParam As Long, ByVal lParam As _
        String) As Long
 
Private Const LB_FINDSTRINGEXACT = &H1A2

Private Sub List2_DblClick()
  Dim nIndex As Long
 
  With Form1.List1
    nIndex = SendMessage(.hWnd, LB_FINDSTRINGEXACT, -1, List2.Text)
    If nIndex < 0 Then .AddItem List2.Text
  End With
End Sub

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