This error happens with sales invoices when a sales invoice in your Xero already has the same number as the document that you are trying to export. Xero requires that each sales invoice has a unique number. 

You can edit the invoice number in Datamolino and export it (for example, add a letter to the invoice number INV0041 >> INV0041a. 

However, you should first check why you have the same invoice number on two different invoices in Xero.