Sub Macro1()
Dim x As Long
Dim y As Long
For x = 1 To Range("C65536").End(xlUp).Row
For y = 1 To Range("F65536").End(xlUp).Row
If Cells(x, 3).Value = Cells(y, 6) Then Message = Message & Chr(13) & "doublons en C" & x & " et F" & y
Next y
Next x
MsgBox Message
End Sub