Petite traduction en VBA...

'déclenche la macro quand une valeur de la feuille change
Private Sub Worksheet_Change(ByVal Target As Range)
    Dim Rng As Range
    Set Rng = ActiveSheet.Range("G2:R2")
    'Vérifie si dans la plage choisie
    If Not Application.Intersect(Target, Rng) Is Nothing Then
        'verifie si la valeur existe déjà dans la plage
        ValueToSearchFor = Target.Value 'récupère la valeur saisie
        If Application.CountIf(Rng, ValueToSearchFor) > 1 Then
            MsgBox "La valeur " & ValueToSearchFor & " existe déjà"
        End If
    End If
End Sub

Private Sub ListBox1_Click()
    ActiveCell.Value = ListBox1.Text
End Sub

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
    Dim c      As Range
    If Not Intersect(Target, [A2:E2]) Is Nothing Then
        With ListBox1
            .Left = Target.Left
            .Clear
            .AddItem
            For Each c In [liste]
                If Application.CountIf([A2:E2], c.Value) = 0 Then .AddItem c.Value
            Next c
            .Visible = True
        End With
    Else
        ListBox1.Visible = False
    End If
End Sub