You cannot have duplicate id
attributes as it means the HTML of the page is invalid, and causes problems in JS.
Use a class to group instead:
<form action="shopping:cart.php" >
quantity 1: <input class="spinner"><br>
quantity 2: <input class="spinner"><br>
<input type="submit" value="Submit">
</form>
Then you need to change your jQuery selector to recognise this class:
$('.spinner').spinner();