show multi select id by js

Answer 1

You are already aware now that id of the element on page should be unique.

When you have multiple elements with same id then id(#) selector returns the first matched element. So as mentioned in other answers you should use class if possible.

By any reason if you can't use the class and want to find with id then you can use it like $('select#jens_id') or $('[id="jens_id"]') and to get the selected option use context to this which will find the selected option for changed select element.

$(document).ready(function() {
   $('select#jens_id').change(function() {
      alert($(this).find('option:selected').text());
   });
});