I just noticed that when a customer places an order. In Step 2 Payment page, if they click on "Confirm" button multiple times, then multiple orders would go through.
The "Confirm" button needs to be disabled once it gets clicked the first time.
Solution/Fix:
Line 279, \tastyigniter\extensions\cart_module\controllers\Cart_module.php
$data['button_order'] = '<a class="btn btn-order btn-primary btn-block btn-lg" onclick="$(this).attr(\'onclick\',\'\'); $(\'#checkout-form\').submit();">' . $this->lang->line('button_confirm') . '</a>';
