This will do what you want provided, as you mention, the spreadsheet is sorted on column A.
This routine does the following:
1. Determine the last row used.
2. Working from the last row used upwards it compares
the value of column A in the current row to column A in
the row above.
if they are the same nothing happens.
If they differ then 2 rows are inserted.
Dim i As Long, LastRow As Long
Application.ScreenUpdating = False
'finds last used row in column A
LastRow = Range("A65536").End(xlUp).Row
For i = LastRow To 1 Step -1
If i >= 2 Then
If Cells(i, "A") <> Cells(i - 1, "A") Then
Rows(i).Insert 'add 1 blank row
Rows(i).Insert 'add another blank row
End If
Else
End
End If
Next i
Application.ScreenUpdating = True