Duplicate content is an issue for all websites and not just a situation where one website copies the information from another website.
In fact, more duplicate content issues come from your own website because of programming glitches that were not foreseen.
Duplicate content happens when information you have on one web page is the same as the information on another page. Of course, if you have a small site with only 10 pages to begin with, you might not be thinking there is a real problem for you.
Maybe there isn't.
However, any website that has any size product catalog might fall prey to duplicate content issues through a number of common issues.
One very common issue is a search box you might have on your site. Search boxes allow your users to search your site for words or jewelry. Want to find all the engagement rings on a site? Simply type in "engagement ring" into the search box and instantly get your results.
Now for the surprise.
That search results page is duplicate content. All those products that are listed 1 time in your product catalog are duplicated on your own search results page. When Google finds duplicate content it has to figure out which is the original and which is duplicate. More work for Google means lower ranking.
Now for the even bigger surprise.
Honestly, in the scenario above Google itself caused the duplicate content, not you. Did you know that Google has the ability to randomly fill in basic forms? We are guessing that they can fill in as many as 5 fields on a form, and we have direct experience with them filling out at least 3 fields.
In one of Google's public training videos, they announced that they will in fact try to fill out forms to discover what's behind them.
Using words and phrases related to your website, they will fill in forms to get the results. Unfortunately the results are duplicate content. It makes you wonder why they would do such a thing and then penalize the little guy for said duplicate content they created.
Solution: Tell your web programmer to add the "Robots=noindex,nofollow" to your own search result page.