All invoices sent through Abillio by default are Advance invoices. Once the payment for the invoice has been received, the client will receive the Final invoice (to the same email address). Therefore client receives two invoices Advance and Final.
Make sure your client understands that and does not pay in double.
If the client prefers receiving a Final invoice, you can ask our support team in the chat window (with the invoice number) to convert your unsent (draft) invoice from Advance to Final.