Multiple duplicate shipping notes. #3

Just doing a code review of a site using your module. Looking at entries in their DB and your code, it seems like you can get multiple shipping notes for an order. You observe the checkout_controller_onepage_save_shipping_method event, which can presumably get triggered multiple times during checkout if people move backwards/forwards in the checkout process. Also it means that the shipping note is created even if the checkout process isn't completed.

Not a big deal just means items get 'stranded' in the database so I thought I'd mention it. I guess the simplest solution would be to store the quote_id on the note, then you could load the shipping node if one exists, else create a new one.