Duplicate content, i.e. the same content accessible from several urls, can have a negative effect on indexing and search engine ranking. In order to push a page forward in search engines like Google, Bing or DuckDuckGo, duplicate content should be avoided as far as possible, because robots have difficulties indexing the same content.
For this purpose, there are different approaches such as "no-index", 301-forwardings, as well as in Joomla the menu entry Alias to link to an article and thus prevent the accessibility of the same content from multiple Urls.
Unfortunately, it is not always technically possible to completely exclude duplicate content in a content management system such as Joomla. In order not to have content indexed twice by search engines like Google or Bing, there is the Canonical Tag in addition to the methods already mentioned. With this Canonical Tag you tell the search engines which links should be indexed or which page has priority for indexing.
"Canonical Tag" different solutions lead to the goal
The Joomla Core already includes the plugin "System - SEF", which often solves existing problems regarding duplicate content and sets the Canonical Tag correctly. In addition to the in-house board resources, the Joomla Extensions Directory has several plugins of different approaches.
My approach is that you only set the Canonical Tag for certain Urls. The links are entered manually into the Joomla Plugin. In most cases you already know for which url you want to avoid duplicate content. Since it is technically possible to exclude some in advance, this Canonical Link List should not be too long. To use this Joomla Plugin, the plugin should be activated after installation and the desired Urls should be entered into the plugin. If you have more than one url, separate them with a comma to ensure functionality.
To check, examine the source code in the head area of the page and you should find the link attribute rel="canonical" here.
<link href="http://meine-joomla-seite.de/mein-erster-artikel-mit-canonical" rel="canonical">
The Google Search Console can also be used to check and show which pages are "Duplicate Content.
Joomla System SEF oder Custom Canonical
With th Joomla Core System SEF Plugin you can already handle "Duplicate Content", but it will not be the desired solution for everyone. Alternatively, the plugin "With Custom Canonical Url", soon available for free download, offers an easy way to set the Canonical tag.
As an experienced web developer from the Stuttgart area I support and advise you in technical SEO optimizations.