What threatens SEO if the site is available with WWW and WITHOUT WWW at the same time?

Question

Search Engine Optimization

I scanned the site through the Screaming Frog SEO Spider and it shows me that almost all the pages of the site are duplicated due to the fact that they are available simultaneously with WWW and WITHOUT WWW, what does this mean for search engine promotion? This is bad?

Answer 1

It threatens that the site can be indexed twice, i.e. a full copy of your site is created.
Check in Yandex and Google which pages in the index are more with www or without www and, accordingly, leave the version of the site that both search engines have indexed more, and then:
1. On the other, put a 301 redirect,
2. make out the site relocation in Yandex.Webmaster
3 . add the correct version to Google Search Console
4. don't forget to create the correct robots.txt and sitemap.xml for the desired version
5. check that the canonical urls are correct (if needed with www, if needed without www)

Answer 2

Doubles Threatens with
violations in the form of doubles