检查重复并仅在表中插入唯一项目，但这仅适用于第1项

I'm creating a shopping cart, I want when I add a product in cart table then only it should insert at one time. So I'm checking duplicate rows using a column product_code in the table, but this only works for 1st item.....If I click another item add to cart button then It do not insert in the table but it shows message item added instead showing item already added....

index page

cart table

action.php

if(isset($_GET['id'])){
        $id = $_GET['id'];

        $stmt = $conn->prepare("SELECT * FROM product WHERE id=?");
        $stmt->bind_param("i",$id);
        $stmt->execute();
        $result = $stmt->get_result();
        $row = $result->fetch_assoc();

        $pname = $row['product_name'];
        $pimage = $row['product_image'];
        $pprice = $row['product_price'];
        $pcode = $row['product_code'];
        $pqty = 1;

        $stmt2 = $conn->prepare("SELECT product_code FROM cart");
        $stmt2->execute();
        $res = $stmt2->get_result();
        $r = $res->fetch_assoc();
        $code = $r['product_code'];

        if($pcode!=$code){
            $query = $conn->prepare("INSERT INTO cart (product_name,product_price,product_image,qty,total_price,product_code) VALUES(?,?,?,?,?,?)");
            $query->bind_param("sssiss",$pname,$pprice,$pimage,$pqty,$pprice,$pcode);
            $query->execute();
            $_SESSION['showAlert'] = 'block';
            $_SESSION['message'] = 'Item added to your cart!';
            header("location:index.php");
        }
        else{
            $_SESSION['showAlert'] = 'block';
            $_SESSION['message'] = 'Item already added to your cart!';
            header("location:index.php");
        }
    }