So what is duplicate content and does your site possibly have it?
Duplicate content is when you have an exact replica of a site or page. The design, the code, the keywords, the links on the page and in the site, the words, etc... Not just a paragraph or two.
Duplicate content exists both on your own site as well as on other sites which scrape everything including your images and even your links.
The reality is that duplicate content is a big issue because it confuses the search engines on which pages or which sites to show. There are a ton of sites out there that have duplicate content on their own sites, but don't even realize it. Canonical links won't always help with those cases.
So how do you know if you have duplicate content? Let's go through a few ways to check.
3 Ways To Check For Duplicate Content
1. Simply grab one or two unique sentences from your site and put it into the search engine. If the exact same sentences are popping up on multiple sites, you have now found duplicate copy (not necessarily pages or sites). You then need to go and figure out the best way to deal with the site that scraped parts of your copy and content, or find a way to make sure your page has more authority.
2. Look in Google Webmaster Tools to see if you have duplicate title tags. This is a great indicator for many things. The first thing it is telling you is to fix your title tags. It also tells you to look at your pages. If your URLs are not redirecting properly, all of the sudden you have created your own duplicate content issue with your own site because your pages are all competing with each other and duplicating themselves. Here is a post I did on URLs for SEO that should help.
3. If you have Affiliates and are sending out a datafeed that comes from your shopping cart, you need to really think about what you are doing. Not only will some Affiliates have more authoritative sites than you, but if you are exporting your feed from your cart, all of the custom descriptions you wrote now exist on your Affiliate's sites and they are now your competition with older sites that may have more authority. Create a second datafeed with it's own unique descriptions so that your Affiliate's don't have your custom descriptions and the copy you worked hard at writing isn't working against your site. This is also true if you are using comparison shopping engines and online marketplaces.
Duplicate content is usually blown out of proportion because people don't understand what it really is. There are plenty of ways to check for it and it is normally one of the easiest things to fix, unless you have a really good SEO scraping an copying you. Then it is up to you to figure out the best way to take care of it.