You could pass the posted information through the query string.
For each form element add another querysting variable. for example if you have form elements named "email" and "name":
$MM_dupKeyRedirect = $MM_dupKeyRedirect . $MM_qsChar ."requsername=".$loginUsername . "&exists=1&name=".isset($_POST['name'])?$_POST['name']:""."&email=".isset($_POST['email'])?$_POST['email']:"";
then, on the bindings panel, for each querystring variable you add, click the plus button and select URL Variable and type the name of the variable.
Next, bind the form elements to the URL Variables.