Not a theme problem...but need htaccess solution for duplicate posts

For some reason, I've quite a number of posts showing 2 urls in Google Webmaster tools. e.g:
/this_is_a_post/1447/
/this_is_a_post/

My permalink structure is /%postname%/%post_id%/

This is my current htaccess

<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

Both types of urls resolve, but to avoid duplicate issues and keep all my links resolvable, I want to 301 redirect all url requests that include the post_id, and just return the postname in the url i.e. /this_is_a_post/

Searched the web but haven't found a htaccess rule that quite works. Appreciate any help.