I have a few invoice errors stating that the invoice number is not unique. Is it possible to export these to Xero?

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.