basic htaccess rules for pretty URLs and avoiding duplicate content (for a filesystem-based URL structure)

.htaccess

<IfModule mod_rewrite.c>
RewriteEngine On

# Redirect secondary domains

   RewriteRule .* - [E=PRIMARY_DOMAIN:________________]
   RewriteCond %{HTTP_HOST}==%{ENV:PRIMARY_DOMAIN} !^(.+)==\1$ [NC]
   RewriteCond %{REQUEST_URI} ^/?(.*)$
   RewriteRule .? http://%{ENV:PRIMARY_DOMAIN}/%1 [L,QSA]

# Strip trailing slash

   # Enforce no trailing slash
   DirectorySlash Off
   RewriteRule ^(.*)/$ /$1 [L,R]
   # Allow no trailing slash (avoid 403 error)
   RewriteCond %{REQUEST_FILENAME} -d
   RewriteCond %{REQUEST_FILENAME}/index.php -f
   RewriteRule .? %{REQUEST_FILENAME}/index.php [L,QSA]
   RewriteCond %{REQUEST_FILENAME} -d
   RewriteCond %{REQUEST_FILENAME}/index.html -f
   RewriteRule .? %{REQUEST_FILENAME}/index.html [L,QSA]

# Hide extensions

   # enforce no trailing index/index.php/index.html in URL
   RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /([^?]*/)?index(\.php|\.html)?(\?.*)?\ HTTP/
   RewriteRule .? /%1 [R,L,QSA]

   # enforce no .php/.html extension in URL
   RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /([^?]*)\.(php|html)(\?.*)?\ HTTP/
   RewriteRule .? /%1 [R,L,QSA]

   # allow no .php/.html extension
   RewriteCond %{REQUEST_FILENAME} !-f
   RewriteCond %{REQUEST_FILENAME} !-d
   RewriteCond %{REQUEST_FILENAME}.php -f
   RewriteRule .? %{REQUEST_FILENAME}.php [L,QSA]
   RewriteCond %{REQUEST_FILENAME} !-f
   RewriteCond %{REQUEST_FILENAME} !-d
   RewriteCond %{REQUEST_FILENAME}.html -f
   RewriteRule .? %{REQUEST_FILENAME}.html [L,QSA]
   
# No server 404

   RewriteCond %{REQUEST_FILENAME} !-f
   RewriteCond %{REQUEST_FILENAME} !-d
   RewriteRule .? / [R=302,L]
   #ErrorDocument 404 /


</IfModule>