Ignore multiple certificate codes (#363)

element/code/classes/element.php

@@ -54,7 +54,7 @@ class element extends \mod_customcert\element {
             $customcert = $DB->get_record('customcert', array('templateid' => $page->templateid), '*', MUST_EXIST);
             // Now we can get the issue for this user.
             $issue = $DB->get_record('customcert_issues', array('userid' => $user->id, 'customcertid' => $customcert->id),
-                '*', MUST_EXIST);
+                '*', IGNORE_MULTIPLE);
             $code = $issue->code;
         }