Order details and email to wrong customer - close time

If there are two customers checking out at the same time (we use Worldpay) the first customer is given the order details and email of the second customer still going through the checkout, this has happened at least twice and always when they are very close together (say 1-2 minutes).

This is obviously a BIG problem and does not give the customer much confidence!!

Any ideas as to what is happening please?

What version of OPEN CART are you using? The only reason that would happen is if the session(s) were malfunctioning.