Mail should be sent only once when Billing Document is Displayed

Hi ,

I have created a ZDriver Program to send email Billing Document to customer when the Billing Document is Displayed , But Here my billing Document is sent evrytime when I click on print preview tab of Biling Document .

SO , here I want to send Billing Document only once when I display the billing document very first , After that PDF attachment should not be sent to customer no matter how many times we display the particular Billing Document .

Is there any flag or condition I can use to resolve this .