Notification Emails but only once!

Notification Emails but only once!

Hi, 

I've been asked to create a notification email whenever a sale has been "Completed" by a contact who falls under a specific account..

That was easy, however now have been asked to only send the notification the first time, and not subsequent sales for example..

  1. John Smith is referred by ABC International
  2. John Smith purchases from ABC International 
  3. Account Manager gets an email from Zoho Notifying them of the above
  4. But no more notifications if John Smith purchases from ABC International again

What would be the best way round this, as I have been going round in circles over the past couple of days, If I did it via the custom module, the account manager will get an email every time they purchase as a new record is created each time?  

If I did it via the contact record and had an autonumber in place everytime a purchase was made, and if greater than one dont send the email?

Thanks

A!