User sees another user's contact/payment info

-- HikaShop version -- : 2.6.3
-- Joomla version -- : 3.5.1

We got feedback from a user who refreshed their shopping cart screen and saw a different customer's information. This is a serious security issue, have you heard of this kind of thing happening before? Any suggestions for fixing it?

User's message:

"I had items in my shopping cart and while proceeding to paypal something went wrong. When I hit the back/refresh button, I saw someone else's contact and payment information on my screen."

Thanks!