This could be caused by two main factors:
if you uploaded an invoice to purchases, exported it, then switched it to sales and re-exported
if the document was previously exported, approved or voided in Xero.
Even though you have encountered this error, the invoice is likely already in Xero. To find the invoice in Xero, please follow these steps:
1. Just click "dismiss" in Datamolino
2. Look up the invoice in Xero using a green Xero icon:
If it doesn’t take you to the right transaction, please search for the invoice number directly in Xero to see if it was sent.