Simply speaking, Duplicate Content is a bad thing. Google does not like it and will penalise you for having duplicate content on your site.
Duplicate Content is also known as ‘copy content’. It’s when your content is on the every page of your website, the same few paragraphs for example. Or it can be when you have taken an article off someone else’s site and used it on your own.
You may think you are doing this to help your customers. However. it will not help the ranking of your own site. You may get dropped to the very bottom of search engine results or even not found at all.
If there is an article on another site you would like your customers / visitors to see then the best thing to do is offer a link through to their site. This means customers can see the article without risking your sites ranking.
You can also re-word articles so they are not the same. It is rumoured that the level Google currently allows is 40% copied. However, the other 60% must be original. I recommend you work to 20% so if the Google Algorithms do change. At least then you will not have to get a content writer to write all the content again.
The best site I have found to check for duplicate content is ‘CopyScape’. You copy and paste the article. The tool tells you how much is copied and which parts you may want to re-write.
I sometimes find that high ranking sites have content stolen off their site. If the other site has been around longer or updates more Google thinks it is the other companies content and you have copied it off of them. If Google is unsure who’s the content is it may penalise both sites. It is recommended that you regularly check your site for duplicate content. Re-write where you can.
It is easier to get your site in order before your site is penalised so you can keep your ranking position instead of being dropped to the bottom and then having to crawl your way back to the top again.
Sometimes it is wise to use an affordable content writer who works outside the industry. They will write the article in a different way anyway and then it is less likely to be duplicated content.