QBO Sync Error: "Duplicate Document Number Error : You must specify a different number. This number has already been used."
In your QBO there is already an invoice with the same Invoice Number for this Vendor and thus is flagging as a possible duplicate.
We advise you check in QBO to confirm the invoice is in fact a duplicate. If that is the case you can mark the invoice as exported.
Otherwise if the invoice number was reused in error then you'll need to reset the invoice and make the amendment to the invoice number. Please note that the invoice will need to be resubmitted for approval with this action as you are changing the information on a fully approved invoice.
Guide: How to Mark Invoices as Exported and Reset Invoices