Private Sub Worksheet_Change(ByVal Target As Excel.Range)
If Target.Column = 1 Then 'Vérification de la colonne A
If Application.WorksheetFunction. _
CountIf(Range('A:A'), Target.Value) > 1 Then
MsgBox 'Attention valeur déjà saisie -- '
Target.Value = ''
Target.Select
End If
End If
End Sub