This error means that you already exported this invoice from Datamolino to Xero and approved (or voided) it in Xero. 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 need to re-export documents from Datamolino to Xero, please delete them in both systems and re-upload to Datamolino.
To review what is the status of your document in Xero, just dismiss the error and click the green Xero icon.
If you have multiple invoices with this error, you can dismiss the errors in bulk. Please remember, after this step the invoices disappear from the list of errors ant there is no easy way how to filter them out and reconcile with Xero in bulk accordingly (other than using our search bar and remembering the supplier name/invoice number).