Why do duplicates appear?

Question

htaccess

Hi all!
The site is static, worked stably for a long time.
.HTACCESS

DirectoryIndex index.html
AddDefaultCharset utf-8

<IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteCond %{HTTP_HOST} ^www.site.ru$ [NC]
    RewriteRule ^(.*)$ http://site.ru/$1 [R=301,L]
    
    RewriteCond %{REQUEST_URI} !\.[a-zA-Z0-9]{2,4}
    RewriteCond %{REQUEST_URI} !/$
    RewriteRule ^(.*)$ $1.html
</IfModule>

Page addresses are displayed without .html , but recently Yandex.Webmaster began to receive notifications about duplicate pages with .html addresses. The duplicates themselves in the search through host: I did not find.
Redirects like:

Redirect 301 /partners.html http://site.ru/partners

just closed the pages, without any redirection...
Tell me, what did you do wrong, how could it happen and how to remove the duplicates?

Answer 1

DirectoryIndex index.html
AddDefaultCharset utf-8

<IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteCond %{HTTP_HOST} ^www\.site\.ru$ [NC]
    RewriteRule ^(.*)$ http://site.ru/$1 [R=301,L]

    RewriteCond %{THE_REQUEST} "^\S+ ([^ ?]+)\.html"
    RewriteRule ^ %1 [R=301,L]
    
    RewriteCond %{REQUEST_URI} !\.[a-zA-Z0-9]{2,4}
    RewriteCond %{REQUEST_URI} !/$
    RewriteRule ^(.*)$ $1.html [L]
</IfModule>