Hi,

PHP Code:

 if(!is_page($post_id) && is_single($post_id)){
        $post = get_post($post_id);
        $author_id = $post->post_author;
        $author_posts_rss_link = get_author_feed_link($author_id);
        $author = the_author();
        return $content.'<div id="subscribe_to_author_posts_feed"><a href="'.$author_posts_rss_link.'"><img src="'.$path.'http://www.cricpak.5gigs.net/wp-content/themes/wpsn/images/feed.png" width="10" height="11" border="0" alt="RSS" title="RSS" /></a><font size="1" face="Verdana">&nbsp;Feeds : '.$author.'</font></div>;
    }

    return $content;
} 


The above php code is showing '.$author.' twice. If I change the position (take it above others) of $author = the_author();, it shows author name more than twice. Is there any solution to finish it after $author = the_author();.

Is this from wordpress?
I think the_author(); function is meant to print authors name, not for returning it. You could use get_the_author(); instead.
http://codex.wordpress.org/Template_Tags/the_author

yeah, it is wordpress. thank u very much.

As a side note, to get a form to stop entering data once the Submit button is pressed, (if that's what you want), you can use this javascript:

<input type="submit" value="Submit" onclick="javascript: this.disabled='true';" />