How to remove a repeating title in wordpress rss

In the directory /wp-includes/ find the files feed-atom.php, feed-rdf.php, feed-rss.php, feed-rss2.php and look for a line in them:

<title type="text"><?php bloginfo_rss('name'); wp_title_rss();?></title>

remove from it:

bloginfo_rss('name');

or:

wp_title_rss();

Done.

See also:
Eliminating duplicate headers on WordPress pages