Since Google is returning these URLs in their search results, it's unlikely that they're considering them to be duplicate content. As indicated here, if Google finds duplicate content on your site they:
do a good job of choosing a version of the content to show in [their] search results.
If you'd like only one of the URL's to be indexed though, as suggested there you can add a rel="canonical" link element to the other pages to indicate which of the URLs you prefer to be indexed.
Alternatively, you could specify how Google should handle parameters (select the "No URLs" option), or use 301 redirects to point the other URLs to the main one.
Sometime after re-crawling your site, Google should drop the extra URLs from your index, but it could take weeks. If they're not impacting your SERP or traffic however, you might just leave things as they are.