Incorrectly configured the return of HTTP-code and 2 more errors on Joomla

Yandex writes:
Probably, the site https://board.rusknr.ru incorrectly configured to return the HTTP code 404 Not Found, which can adversely affect the indexing of the site by the robot. Configure the 404 code to be returned to request pages that do not exist.
Some pages of the site duplicate each other - you need to configure. It is possible to prevent the index of unnecessary pages.
The pages have the same titles and descriptions - you need to customize. It is possible to prevent the index of unnecessary pages.
The site is on Joomla. You need to set it up. Interesting low cost.