The definition of Duplicate Content.
Your content's flagged as duplicate content by search engines when it's found on other pages within your own domain or found on someone else's website. I'd also like to add that the content doesn't need to be identical in order to be considered a duplicate, similar is enough.
why do i need to avoid duplicate content?
The first reason is copyright infringement. If the content's not yours, that is, if you've pasted it from another website without the author's permission and credit, it's considered malicious (and illegal) -- even if it's accidental or you didn't know.
A second (more common) reason's that you need to have that content on multiple pages. Now, this one is unavoidable in some cases, and down below I'll tell you how to do this the right way, but let me first give you some scenarios where duplicate content is needed.
A third (unfortunately common) reason duplicate content is considered a bad practice is because it's used to manipulate a person's organic ranking within search engines. I won't get into any deep details on how that's done, but here's how it generally is: in the past, people and SEO companies used to duplicate your content across different domains to ensure you would get a high organic ranking for a particular set of keywords. This was before Google started actively punishing sites for doing this. Now-a-days, tricks like this massively negatively affect your organic ranking.
a few scenarios where duplicate content might be required.
You might have an eCommerce store and are selling items that have headings and titles which need to stay the same across webpages. One example is the homepage, where you're featuring your product with a title / description, the shop page which lists your product beside other products with a duplicate title / description, and the product page that features the duplicate title / description and some additional information with a "buy" button.
You might have a webpage that needs to be printed off without the flashy style(s) typically found on webpages. So, you make a printer-friendly or printer-only version of it.
Or you generated a more mobile-friendly version of your page with stripped down content.
In scenarios like these (and I'm sure I've missed a few others) duplicate content is nearly unavoidable. Luckily, there's an proper way to handle this.
deal with duplicate content with canonicalization.
If you do have legitimate reasons for having duplicate content from any source on your domain, you can use something called canonicalization. The hard-code you would use is rel="canonical" and the URL of the original page on the duplicate(s), signaling to search engines like Google that "Yes, this is duplicate content, but here's the original". This prevents search engines from punishing your overall organic rank.
That's about it!
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