Upon placing an order the sale is registered in admin under “orders” and issued an order number. If I then make a second order on the same device, PC or mobile, and go to checkout the system has remembered my details and goes straight to (step 2) make payment. Once the second sale is complete the first order in admin is overwritten by the second order.
For example; the order number 20076 which was issued with order 1 is overwritten with the details of order 2.
The above is when you checkout as a guest. If logged into your account everything seems to work okay as any additional orders placed always go to step 1 of the purchase process and not step 2 as with a second order from a guest.
One solution would be to automatically clear cookies once you you have reached the checkout/success page. The problem here is that logged in customers would be automatically logged out.
I have noticed that if you leave it around 10 minutes the form fields in checkout have been cleared and the order is allocated its own invoice number. The problem only arises when additional orders are placed by guests soon after the initial order.