I copied and pasted the code after line 15 of the checkout.php as suggested. The result was that when the "PayPal" button was selected the page basically reloaded to itself without going to PayPal.
So I deleted the added code and then when the "PayPal" button was selected the site would go to the checkout page with a header error at he top.
I replaced the checkout.php file with a fresh copy then when I selected the "PayPal" button I was directed to the paypal site. I did not go further because PayPal will not let me pay myself.
The original problem DID NOT occur from using the back button but rather from a failure to log into paypal and selecting the return to site option on the paypal login page.
I was going to leave the code in the checkout.php file you suggested, but did not so you can checkout my other problem posted in my other thread called "checkout problem".
I appreciate any help offered.