Echo post title in post

Use the_title():

function add_post_content($content) {

        if(!is_feed() && !is_home()) {

                $content = the_title( '<p>', '</p>', FALSE ) . $content;
        }

        return $content;
}

add_filter('the_content', 'add_post_content');

The first two arguments are for $vefore and $after. If a post doesn’t have a title, you get not extra markup. The last argument makes the function returning the string. Otherwise it would print it out immediately.