If you receive the publishing error Duplicate Document Number Error, this means that the invoice number already exists in your QuickBooks Online account.
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Open your QuickBooks Online and navigate to Settings >> Account and Settings >> Sales.
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Check the Custom transaction numbers option is turned OFF. If the setting is ON, select the edit button and select the Custom Transaction Numbers checkbox.
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Back to QuickBooks Online Settings >> Account and Settings >> Advanced.
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Check that Warn if duplicate check/bill number is used is also turned OFF.
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Then re-export the transactions that did not get into Quickbooks because of the error.
Newer versions of QBO:
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