Using the code you provided gives me error 500.
Thats the only thing with htaccess, there are so many different ways of doing the same redirect, and when you don't have the site in front of you, you don't know which ones are going to work and which one will break the site.
try this one: (if this one doesnt work, Google 'redirect trailing index file' and you will find other way to do it)
RewriteCond %{HTTP_HOST} ^www\.
sitenamehere\.com$ [NC]
RewriteCond %{QUERY_STRING} ^$
RewriteRule ^index\.html$
http://www.
sitenamehere.com/? [R=301,NE,NC,L]
Activate mod_rewrite on your server. Mostly you can do this by editing the php.ini - if you a re on a shared host, just ask you provider.
If his other redirect is working, this must already be on.
Yeah, I got a redirect using .htaccess
RewriteEngine on
RewriteCond %{HTTP_HOST} ^([a-z.]+)?website\.net$ [NC]
RewriteCond %{HTTP_HOST} !^www\. [NC]
RewriteRule .? http://www.%1website.net%{REQUEST_URI} [R=301,L]
don't want to state the obvious, but this is actually working yeah?
Does this issue conflict with SERP? Because in search results they appear correctly...
Not right now as its not in the SERPS, but trailing /index.html duplicates of the home page, and duplicates such as 'www' and 'non-www' can get indexed causing duplicates content, and link juice can be lost if links have been built to different versions that are in SERPS. So yeah a redirect should be used, or canonical if you cat get the redirect working.