make #skip_duplicate_check useful

right now the FAPI attribute #skip_duplicate_check can only be manipulated via hook_form_alter() -- it cannot be passed as an initial attribute in a form array, because drupal_prepare_form overwrites the value after the initial form array is passed in. this is rather inconvenient if you actually want to use this attribute. :)

attached patch simply adds a check for an existing setting before setting to the default value.