Form disappears on submission Shopping cart - php

Question

So at the moment I am trying to create a shopping cart using PHP
currently I'm having trouble submitting a form which adds a product to cart. So essentially I have linked up my database to the my shop.php page, so it loops through each item in the DB and displays the product. Each product has a form which uses POST which when submitted, adds the item to the cart SESSION.
So at the moment, when I click the 'add to cart' button the item is added and works fine. But when i click 'add to cart' for the same item again, the entire product gallery stops working. The issue only occurs when I try to add an item that's already been added. I feel like the issue revolves around submitting the same form twice but I am not quite sure.
<div class="container">'
<div class="box">'
<?php
$query = "SELECT * FROM items";
$result_set = mysqli_query($link, $query);
if (mysqli_num_rows($result_set) > 0){
while($row = mysqli_fetch_array($result_set)){
?>
<div class="displayCase"><br><br><br>
<form method="POST" action="shop.php?action=add&id=<?php echo $row['id']; ?>">
<img class="phone1" src="data:image/jpeg;base64,<?php echo base64_encode($row['image'] )?>" />
<h1> <?php echo $row['name'] ?> </h1>
<h2> <?php echo "$" . $row['price'] ?> </h2>
<label id="QTY">QTY </label><input class="quantity" id="id_form-0-quantity" min="0" name="form-0-quantity" value="1" type="number"><br />
<select class="phoneType">
<option value="X">Select Device</option>
<option value="X">iPhone X/XS</option>
<option value="X">iPhone X/XS Max</option>
<option value="X">iPhone 11</option>
<option value="X">iPhone 11 Pro</option>
<option value="X">iPhone 11 Pro Max</option>
</select><br />
<input type="hidden" name="name" value="<?php echo $row['name'] ?> ">
<button type="submit" name="add" value="addToCart">Add To Cart</button>
</form>
</div>
<?php
}
}
?>
</div>
</div>
Cheers
EDIT:
Okay so for some reason it seems to take a full reload after the form is submitted for it to work. So I tried this and it seemed to fix it. Still unsure why this is the case, and whether this is a clean way or if there is a better way of doing it
if (isset($_POST['name'])){
header("Loction: redirect.php");
}
then I created a redirect.php page redirects back to shop.php