Sub ESSAI()
Dim T, D, i&, e
T = Range("A1:a" & Cells(Rows.Count, "a").End(xlUp).Row)
Set D = CreateObject("scripting.dictionary")
For i = 2 To UBound(T): D(T(i, 1)) = D(T(i, 1)) + 1: Next i
For Each e In D.keys: D(e) = IIf(D(e) = 1, 0, D(e)): Next e
For i = UBound(T) To 2 Step -1
If D(T(i, 1)) > 0 Then _
D(T(i, 1)) = D(T(i, 1)) - 1: T(i, 1) = T(i, 1) & "_" & (D(T(i, 1)) + 1)
Next i
Range("A1").Resize(UBound(T)) = T
End Sub