MDL-63183 auth_shibboleth: Don't render page when already logged in

auth/shibboleth/login.php

@@ -62,7 +62,19 @@
     $PAGE->set_heading($site->fullname);
 
     echo $OUTPUT->header();
-    include("index_form.html");
+
+    if (isloggedin() and !isguestuser()) {
+        // Prevent logging when already logged in, we do not want them to relogin by accident because sesskey would be changed.
+        echo $OUTPUT->box_start();
+        $params = array('sesskey' => sesskey(), 'loginpage' => 1);
+        $logout = new single_button(new moodle_url('/login/logout.php', $params), get_string('logout'), 'post');
+        $continue = new single_button(new moodle_url('/'), get_string('cancel'), 'get');
+        echo $OUTPUT->confirm(get_string('alreadyloggedin', 'error', fullname($USER)), $logout, $continue);
+        echo $OUTPUT->box_end();
+    } else {
+        include("index_form.html");
+    }
+
     echo $OUTPUT->footer();