We could also sort the merged arrays with the sort_merge_arrays setting, and remove the data that matches knockout_prefix. An array member or entire keys can be removed from the resulting hash:

 lookup({
   'name'  => 'classification',
   'merge' => {
     'strategy'          => 'deep',
     'knockout_prefix'   => '--',
     ‘sort_merge_arrays’ => true,
   },
 })