Range("C11")=E, Range("E2")=E, mais C11<>E2 !!

Sub an_inputbox() 
Dim inputbox_adding As String
debut:
inputbox_adding = InputBox("ajouter les données")
If UCase(inputbox_adding) = "STOP" Then exit sub
if application.worksheetfunction.countif(range("C:C"),inputbox_adding)>0 then
    msgbox "Reférence deja existante"
else
    range("C65536").end(xlup).offset(1,0) = inputbox_adding
end if
goto debut
End Sub

Sub an_inputbox()
    Dim c As String
    Dim ws As Worksheet
debut:
    c = InputBox("ajouter les données")
    If c = "" Then Exit Sub
    If UCase(c) = "STOP" Then Exit Sub
    If WorksheetFunction.CountIf(Range("C:C"), c) = 0 Then
        Range("C65536").End(xlUp).Offset(1, 0) = c
        Set ws = Sheets.Add(after:=Sheets(Sheets.Count))
        ws.Name = c
    Else
        MsgBox "Reférence deja existante"
        GoTo debut
    End If

End Sub