An address like domen.ru/... gets indexed - repair or redirect?

Question

Search Engine Optimization

Question about working with links like: domen.ru/.... -- i.e. we can prescribe after domen.ru/...

The meaning is in .... after the slash. For some reason, search engines sometimes index these pages, and they are duplicates of the main page.
What is the best way to do this?

Do I understand correctly that
In the worst case - a server error
In the average case: will return the main page
In the best case: 404 error.

Examples:
https://www.mvideo.ru/... - 404 page not found
https://www.ulmart.ru/... – opens the main page of the site
https://www.dns-shop.ru/ ... - page not found

Answer 1

For such urls, a 301 redirect should be done on the server, after a while the pages will fall out of the index.

Answer 2

We understand ....

Worst case - server error

- Whoever made the website is an idiot. This means the program is crashing. Analog - Close program, BSOD. There should be no denial of service.

Medium: Returns the main page
. Best: 404 error.

- At your discretion. There is no page. You define what should be shown to the user:
a) The page that the user has landed on a non-existent link.
b) Main or other page (password entry page).

For some reason, search engines sometimes index these pages, and they are duplicates of the main page.

Here we need "SEO optimizers" (exactly in quotes). It's great when your site is on 100 pages of a search engine and gives the same thing, and if on the first pages, then the user will never reach competitors)))))). For example, for search engine robots and for users to issue different content. As a user, I will scold such a site and lower my karma))).