This error dictates that this invoice name/number/ID is already being used with Quickbooks.
In other words... Whatever is listed in the ID column for the invoice in question, is already the name for an existing invoice in QBO.
The quickest solution would be to adjust the invoice ID/name to remove the duplicate. However, if you would prefer to NOT adjust the invoice number, you can turn off the notification that is producing the error. :)
Here are the steps to turning off the notification:
1. Open your QuickBooks Online and navigate to Settings >> Account and Settings >> Sales
2. Check that 'Custom transaction numbers' is turned OFF
3. Click back over on the left menu panel to QuickBooks Online Settings >> Account and Settings >> Advanced
4. Check that 'Warn if duplicate check/bill number is used' is also turned OFF
Please make any of the above adjustments and then retry syncing.
If you have followed all the steps and you are still experiencing the same sync error, please contact us via the Chat Bubble in the bottom right.