Users on my server have been receiving spam, consistently, for some time now. I have most of spamassassin's plugins enabled, and I have made sure to enable verbose logging, where I can see that all of the plugins are working.

Why is it, then, that my users are able to receive the same exact junk mail several times in a day without the message being flagged in some way?

Here are the relevant headers of an email that I personally have received several copies of:

X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on
  sub.domain.tld
X-Spam-Level: ***
X-Spam-Status: No, score=3.0 required=4.0 tests=BAYES_50,FSL_HELO_NON_FQDN_1,
  HELO_NO_DOMAIN,HTML_MESSAGE,RCVD_IN_BRBL_LASTEXT,RDNS_NONE autolearn=no
  version=3.3.1

Have I accidentally missed the plugin that can see that I, and others, have received the same message multiple times in a short period of time?