Interdire l'entrée d'une valeur identique

Set c = Cells.Find(What:=Target.Text, LookIn:=xlValues, LookAt:=xlWhole)
   If Not c Is Nothing And c.Address = Target.Address Then Set c = Cells.FindNext(c)
     If c.Address <> Target.Address Then
       If c.Column = 256 Then

Private Sub Worksheet_Change(ByVal Target As Range)
Dim c As Range
If Target.Count > 1 Then Exit Sub
If Target.Column = 3 Then
 If Target <> "" Then
  If Len(Target) <> 24 Then MsgBox "Y a pas 24 chiffres dans le code!": Target = "": Exit Sub
  If Val(Target) = 0 Then MsgBox "Ce code n'est pas un nombre!": Target = "": Exit Sub
'On colle dans la colonne IV le contenu de la colonne C de la feuil4
  Sheets("Feuil4").Columns(3).Copy Range("IV1")
   Set c = Cells.Find(What:=Target.Text, LookIn:=xlValues, LookAt:=xlWhole)
   If Not c Is Nothing And c.Address = Target.Address Then Set c = Cells.FindNext(c)
     If c.Address <> Target.Address Then
       If c.Column = 256 Then
         MsgBox "Ce code existe déjà en feuil4!C" & c.Row: Target = ""
       Else
         MsgBox "ce code existe déjà en cellule " & c.Address(0, 0): Target = ""
       End If
     End If
   End If
'puis on efface la colonne IV
  Columns(256).Delete
 End If
End Sub