On the SOP daybook there is a Dispatched Column which shows the amount already dispatched and then there is a short column which tells you what is left on the Sales Order. Look at the column "Yet to be invoiced" below. This will show how many items have been invoiced. If you enter an invoice in the Normal Daybook it will not be logged here so you could end up sending out invoiced twice. Always select items that you want to invoice and then click on the Create invoice button at the bottom of the SOP Daybook. This also has the added advantage of creating one invoice for several orders.