Option Explicit
Sub numeroter_lignes()
Dim C As Range, i As Integer
i = 1
For Each C In Range("A1:A" & Range("A" & Rows.Count).End(xlUp).Row)
C.Offset(, 1).Value = i
If C.Value = C.Offset(1, 0).Value Then
C.Offset(, 1) = i
i = i + 1
Else
i = 1
End If
Next C
'tri croissant
Range("A1").CurrentRegion.Sort Range("B1"), xlAscending
End Sub