If a website has been around for a while, there may be different versions of it in the index. Versions of a website may look something like this: http://example.com, https://example.com, www.example.com, and example.com.
Consequences of multiple website versions
In short, http/https/www/non-www are the different versions of a website and potential duplicate content. If a website with several of these versions is in the index, Duplicate Content can occur. This in turn can lead to a loss of ranking.
This is especially relevant in search engine optimization to identify and avoid the so-called duplicate content.
Thematically relevant sub-pages: