If you have duplicate content on different pages of your website, search engines won’t know which one to index. The correct URL, which you want to be indexed, is known as the ‘canonical’ URL.
You can inform search engines of which version is the canonical URL by using a canonical tag in a piece of code which looks like this:
<link rel=”canonical” href=”www.example.com” />
The canonical tag can be placed in all versions of a webpage to tell search engines which URL is the canonical URL to be included in the index.