On my copy of checkout.php at line 383 there was the following:
(if you have been here before <a href="users_LogIn.php?accesscheck=checkout.php">Log In</a> to save time)<?php echo(ValidatedField("confirm","confirm")); ?>
Attached to the login in the header.
At line 491 was the following:
<div class="text-indent">Is this the wrong account? Please <a href="users_LogIn.php">log in</a> or <a href="users_Registration.php">register</a>.</div>
After reading what Ray said it seemed to me that
"<a href="users_LogIn.php?accesscheck=checkout.php">" as found in the first instance was what he meant so I modified the second instance to read the same.
That made no difference.
Than reading again I saw that you both had "accessCheck" rather than "accesscheck" as it is found in my copy of that page. Thinking that may be the problem I changed case in both locations and still no change. I'm still taken to users_Profile.php