How can we improve WHMCS?

When using the Email Marketer to send follow up emails for product purchases, it will send the email once per product owned. There should be an option to send only once per client. For example, if my client has "Product X" they have purchased 5 copies of, it should not email my client the same email 5 times. Adding this option will help eliminate a lot of spam issues and if you're using an SMTP service based on credits it will not eat away at them.