First of all the original author - David Pollak - is a JCG (Java Code Geek) thus is a part of a Java developer community that is committed to share their knowledge and experiences so as to help each other. As with all of our JCG members we have granted full privilege to republish and promote every article they write that we believe is valuable to the rest of the JCG community.
Furthermore we (the Java Code Geeks community founders) are Java developers ourselves and have contributed first (and still contributing) with high quality articles. That is the reason that we do not want to simply maintain a link aggregator site but a site that will host the most valuable content for the Java developers community.
Now to your question ...
Just because by submitting the link pointing to the reposted article (hosted on Java Code Geeks) and not the one pointing to the original author's blog we promote not only the specific author's work but the work of all the Java Code Geeks community; since our site is the entry point for them all. That is our obligation towards all our community members!
As far as content duplication is concerned I just wanted to pinpoint that in the majority of cases we do not just copy and paste content; We perform lexical and syntactical corrections (since many of our JCG partners do not speak English as their prime language) along with article and code snippets formatting so as every article to be as readable as it gets.