If your website is built with Joomla, chances are that Google is indexing far more pages from your website than actually exist. I won’t dig into the core technology of Joomla to diagnose what causes this but, simply, the structure of Joomla and its various extensions can mean your pages end up with multiple URLs.
There’s the core Joomla URL structure, then there’s the friendly URL you tell Joomla to present, and each of those may have two or three variants, meaning you can actually access the same page using different links.
The problem there is that the search engines, once they discover those links, can index pages multiple times, creating a duplicate content problem you want to avoid.
If you are using Joomla the only way to combat this is to keep a watchful eye on your Google Webmaster Tools account or use a program like Screaming Frog to monitor for any duplicate meta titles or meta descriptions notices. If you have the sh404sef component installed or, better still, aceSEF, you can create redirects in the CMS to point the multiple URLs to one master URL.
Another challenging “feature” of Joomla (I say with clenched fist) is when you try to upgrade something. There’s always a risk that the URL structure of the site can change, so you may suddenly find that the 5000 URLs indexed by Google are broken and Google Webmaster Tools starts getting hot under the collar about it.
As well as creating new fresh content on a continual basis, an important part of SEO is maintenance, and Google Webmaster Tools is a good port of call for this.