In Magento 2.0.8 in cart page if you click more than once Processed Checkout button cart getting empty but it will not reproduce 2.1.0 and above
its Magento default issue we can fix this issue by replacing SessionManager.php of 2.1 version
to get click below link
and replace SessionManager.php of Magento 2.0.8
cart will become empty if user click processed checkout button in cart page more than 1 or 2 times continuously before loading the page.
please check the code changes from 2.1.x