This is an error generated in Loblaw's system regarding an issue with a line item you have sent to them recently in an Invoice EDI document. This error states that you have transmitted a duplicate invoice to Loblaw. This means that Loblaw's system has processed two invoices with the exact same invoice number twice.
It's very important to note that your invoice will be always get rejected if it uses an Invoice number that Loblaw has already received, even if the first version was itself rejected. To avoid getting your invoice rejected again, we recommend adding a small letter a to the end of your invoice number so that you can use the same invoice number for your records but at the same time make it possible for Loblaw to accept the correction.
If you did not send the same invoice twice, this could be due to some kind of technical issue causing two invoices to be processed at Loblaw's end, even though you only sent one out of your WebForms account. If this is the case, we would advise you to contact Loblaw just to make sure that they have processed at least one copy of your invoice.