Duplicate content is a problem that could totally ruin your website. Apparently many web programmers still don't understand the cause and effect of duplicate content and even brand new website fall pray to the disasters of duplicate content.
We recently reviewed a website that did a full redesign and installed a new content management system. Last year, this website had fantastic ranking for almost all of their target keywords. Unfortunately, with the new design and CMS, they also suffered a massive duplicate content problem.
Each of their products had at least 6 identical pages created using different URLs. It seems like Google penalizes this type of sloppy programming by completely removing the website from the search results.
After our review and findings, this jewelry store is trying to correct their duplicate issues and they are already seeing an increase in organic traffic for the pages they've fixed.
There are other types of duplicate product content that will not get you so seriously punished.
An example of non punishable duplicate content would be a page of sale items. You might add a web page dedicated to all your sale items. Those same items are also in your regular product pages, but Google publicly states that they are able to understand this specific example and will not penalize you for it.
On the other hand, if you have a standardized sales message of 100 words, you will get penalized if you include that message on the footer of all your landing pages. If you are reading between the lines here you should be realizing that duplicate content is not only based on entire pages, but on sections of pages.
In conclusion, duplicate content is really bad. Try not to include anything duplicate on your website and you'll be starting out your SEO efforts correctly.