save post running twice and creating two database entries – how do I disable this?

Try moving your

 if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
  return;
 }

into your rdf_argument_function(). Any change?

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)