Private Sub Worksheet_Change(ByVal Target As Range)
Dim plage As Range, c As Range
Set plage = Range("A:A") 'à adapter
Set Target = Intersect(Target, plage, UsedRange)
If Target Is Nothing Then Exit Sub
Application.EnableEvents = False 'désactive les évènements
For Each c In Target 'si entrées multiples
If c <> "" Then If Application.CountIf(plage, c) > 1 Then MsgBox "Doublon en " & Target.Address(0, 0) & " l'entrée va être annulée...", 48: Application.Undo: Exit For
Next
Application.EnableEvents = True 'réactive les évènements
End Sub