Help avoiding duplicates when adding to PHP Session

When trying to add one or more arrays to my Session array, I run into a problem where the added array is duplicated on a page reload. I've tried a few different things to avoid this, but can not seem to find the solution.

Is it possible to somehow count how many times the page has been refreshed so that $discountedItems is only added once to the Session, regardless of how many times the page get's refreshed?

$discountedItems = array(
                    'name' => 'discount', 
                    'quantity' => '1', 
                    'price' => '-35.00',
                    );

array_push($_SESSION['items'],$discountedItems);