jquery serialize and multi select dropdown
Try changing the name of the select to factoryUsers[]. That way you can loop through it in your backend.
The string output you've described above is the correct way of submitting multiple values for forms with the same name over HTTP, so jQuery is working correctly. It's up to you to handle how this is processed on the server-side, which is then dependent on what language you are using.
If you're using PHP, this may help: http://bytes.com/topic/php/answers/12267-how-php-_post-gets-multiple-values-html-form
Can you tell us what language you're using?
All you need is change name="factoryUsers" to name="factoryUsers[]" PHP will treat it as array.