need to edit some features in my website

Avg Bid (USD)
Project Budget (USD)
$10 - $30

Project Description:
My website is
I want someone to do the following task on my website:
-after a user adds a product to shopping cart, he cannot add the same product again. If he presses 'add to cart' the same product, then show something like 'you already have this item in your shopping cart!'

Skills required:
Graphic Design, HTML, MySQL, PHP, Website Design
About the employer:
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