Duplicate Content occurs when the same or very similar content appears on more than one unique URL on the internet. This can happen within your own website (internal duplicate content) or across different domains (external duplicate content).
The main problem with duplicate content for SEO is that search engines have difficulty deciding which version of the content is the original or most relevant to display in search results. This can lead to several issues: search engines may not know which page to rank, link equity can be diluted across multiple URLs, and your pages might rank lower or not at all for target keywords. It's not typically a penalty, but rather a ranking signal dilution.
Common causes include printer-friendly versions of pages, URL parameters (e.g., ?sessionid=), HTTP vs. HTTPS versions, or content syndicated across multiple sites without proper attribution. To fix duplicate content, you can use <link rel="canonical" href="..."> tags to tell search engines which URL is the preferred version, implement 301 redirects for old or non-preferred URLs, or use the noindex meta tag for pages you don't want indexed.