Small PHP modifications for a Prestashop script

I have a script/module which add products in a list with products. The problem is that I can add the same product 2 or more times and then the product becomes duplicated. 1. I just need a rule which will not allow me to add 1 product more than once 2... (Budget: $10 - $30 USD, Jobs: HTML, MySQL, PHP, Prestashop, Software Architecture)