Trailing URLs such as ending with index.php or index.hlml is not a big deal, however you may find that you’re sharing the link juice across 2 identical ‘sources’. Best to simply remove the trailing index reference altogether and redirect any traffic to it, to the directory root instead.
RewriteCond %{THE_REQUEST} ^GET\ /.*/index\.(php|html)\ HTTP
RewriteRule (.*)index\.(php|html)$ /$1 [R=301,L]
Job done.
