As long as the invoice is still a draft and has not been approved in Xero, you can update the invoice in Datamolino and re-export the invoice.
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If you re-export an invoice from Datamolino, we don’t create duplicates in Xero. We just update the original transaction. If a document is approved/voided we cannot find the original transaction and you get an error message. In situation like this, it’s necessary to re-upload such document into Datamolino and export again.

This is a precaution to keep your data safe in Xero.