the json type just stores the string value passed to it, but only the final value is available to the JSON-aware operators.

  mydb=# select
      ('{"foo": 1, "foo": 2}'::jsonb)->'foo' jsonb_foo,
      ('{"foo": 1, "foo": 2}'::json)->'foo'  json_foo;
  jsonb_foo|2
  json_foo|2