Quickbooks Error Code (6140): Duplicate Document Number Error
For a full list of Quickbooks Error Codes click here.
This indicates that either a Bill, a Bill Payment, an Interest charge, or maybe something else has not been synced because the QBO Document Number that Syngency is trying to use has already been used by Quickbooks. There are cases where a duplicate number doesn't exist in Quickbooks, but the invoice number is still not accepted by Quickbooks.
Select Bill Syncing
Locate the Error Code by scrolling down
If the above steps don't correct the issue. Modify the invoice number either in Syngency or in QBO first, and then complete the steps listed above. It may be best to make the modification to the Invoice in QBO that is causing the issue as this is not a Customer Facing Invoice. To edit the invoice number in Syngency, complete the following steps:
Open the Invoice
If there are any payments, select Delete in the top right hand side of the screen You will not be able to edit a paid bill until payments are deleted first.Please make a note of these payment details before deleting it so that they can be reapplied once the invoice number has been changed.
Update the invoice number by retyping a higher number in the ID# field
Select Save invoice
To change an invoice number in QBO follow the steps below:
Go to Sales or Invoicing, then click on the Invoices tab.
Select the invoice in question.
Go to the Invoice no. field and change the invoice number.
Click Save and Close or Save and Send.
Finally, if the above steps still do not resolve the issue, try deleting the invoice and re-generating.