This error happens with sales invoices when the invoice in your Xero already has a number 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 these duplicate invoice numbers on two different invoices.