> (though I'm not sure how gron handles that situation).

It seems both gron and jq only use the value that has been defined last:

  ~  echo '{"a":1,"a":2}' | gron                                                                                                                                   
  json = {};
  json.a = 2;
  ~  echo '{"a":1,"a":2}' | jq                                                                                                                                    
  {
    "a": 2
  }