bloquer saisie doublon

Private Sub Worksheet_Change(ByVal Target As Range)
  If Not Intersect(Range("monchamp"), Target) Is Nothing And Target.Count = 1 Then
    For Each c In Range("monchamp")
       If UCase(c.Value) = UCase(Target.Value) And c.Row <> Target.Row And c.Value <> Empty Then
         réponse = MsgBox("Doublon en :" & c.Address & Chr$(10) & _
           "Voulez-vous le garder ?", vbYesNo + vbInformation, "DETECTION DOUBLON")
         If réponse = vbNo Then
            Application.EnableEvents = False
            Target.Value = Empty
            Target.Select
            Application.EnableEvents = True
          End If
        End If
      Next c
    End If
End Sub