var $inputs = $('[name]'), names = [], namesDuplicados = [];

$.each($inputs, function(index, value){
  var $this = $(value);

  if(names.indexOf($this.attr('name')) != -1){
    namesDuplicados.push($this.attr('name'));
  }

  names.push($this.attr('name'));
});

console.log(namesDuplicados);