Content duplication issue with dynamic URLs

This issue is not necessarily with wordpress, rather Google allows you to exclude specific url parameters.

So you should exclude the session_id parameter, and possible others… have a look around the Craw->url parameters section of you webmaster account. There are a number of different options, depending on what the roles of your parameters are.

https://www.google.com/webmasters/tools/crawl-url-parameters?