Set derlign = Range("B" & Rows.count).End(xlUp)
'MsgBox ("Dernière ligne " & derlign & " !")
Set r1 = Range("A2:A100")
Set r2 = Range("B2:B100")
For N = 2 To r2.Rows.count
    If r2.Cells(N - 1, 1) = r2.Cells(N, 1) Then
       r1.Cells(N, 1) = r1.Cells(N - 1, 1)
    Else
       r1.Cells(N, 1) = r1.Cells(N - 1, 1) + 1
    End If
Next N
End Sub`

N°  REF
1   305-77-871
2   402-88-920
2   402-88-920
3   406-55-585
3   406-55-585
3   406-55-585
4   404-11-885
4   404-11-885
4
4
4
...