SAP automated invoicing - duplication of invoices sent via email

Hi

Here's the issue:

After an invoice is released to accounting via VF02, for some reason the automation set up sends (sometimes and in random customers) via email one blank invoice, then complete and correct invoice and then another one.

We have not been able to figure out why does that happen and what is triggering it.

Can anyone provide any ideas please as to what it can be?

Thanks