How does duplicate content affect indexing?

Question

Search Engine Optimization

Hello.
I came across a statement that duplication of pages has a bad effect on indexing by search engines. But I don't quite understand how bad it is.
For example, there is this:
site.ru - the main page;
and
site.ru/abracadabra - there is no such page, but the engine throws it on the main page.
Poke, please, the link, where it is written about it in more detail. And then in the search it throws out answers from the category - "It's bad, because it's bad."

Answer 1

The search engine negatively perceives duplicated pages and lowers their rating in the search, even worse if it works just like
site.ru/porno_s_jivotnimi for
your competitor, it will be enough for your competitor to create such links on the Internet, index them and there is a big chance that your site will snatch out a ban.
What is good and bad can be read in the official guides from Google and Yandex for sites.