Looping and creating records without a duplicate.

I have a product table and a buyer table. For me to get the count of purchase made by a buyer I did create a max summary value in the relationship between the two. in the buyer table I can click on the summary value to see the chronology of the purchases. I want to record the purchases in another table by first looking if the buyer has bought more than 3 numbers of products for them to be added in the table. How do I go about this by ensuring I don't have duplicate records an also having this run to add a record when a customer makes another purchase?