Backbone leading to duplicate copies of data

backbone-duplicate-data.js

var Post = Backbone.Model.extend({

    parse: function( resp ) {
        return _( resp ).extend({
            created_by: new User( resp.created_by )
        });
    }

});

var post1 = new Post(),
    post2 = new Post();

post1.get( 'created_by' ).id === post2.get( 'created_by' ).id; // true
post1.get( 'created_by' ) === post2.get( 'created_by' ); // false