This means that you previously exported this invoice to Xero and approved (or voided) it there. We do not overwrite approved (or voided) invoices in Xero. Only documents that are still in Drafts can be re-exported from Datamolino.
This is a precaution to keep your data safe in Xero, we try not to create duplicates when someone exports data multiple times.
If you still wish to re-export this document, please delete the document in Xero and Datamolino and re-upload it again.