There's a problem in the way the path gets handled. I tested on two drupal 7 installs.
If you send from http://domain/node/5 the path in the log appears as http://domain/node/dupeoff/check without the nid and this is the error:
Notice: Undefined property: stdClass::$nid in dupeoff_check_api() (line 31 of sites\all\modules\dupeoff\dupeoff_api.php).
The referrer appears correct in the log but the location is a different story