Let's say you have a very dynamic web site and you also generate your meta keywords dynamically. Could you be penalized (or receive a warning) if those meta keywords are very similar? In a nutshell, yes, though there's a 'debate' across two forum threads.
You may receive a warning in Google Webmaster Tools, but you will probably not receive a penalty. However, as JohnMu says in a Google Groups thread, those "warnings" are mostly nudges that you should try to diversify the meta data. You should keep in mind that the terminology should "makes sense to your users." If that means you need to work with duplicate meta data, then that's how you should proceed.