Avoiding Duplicate function names in Post Loop (WP_Footer script)

I would wrap the whole thing in a class and put your data into a class var.

class WPA69616_Plugin {
    private $data="";

    public function __construct() {
        add_shortcode('my_shortcode', array($this, 'add_content'));
        add_action('wp_footer', array($this, 'output_content'));
    }

    public function add_content($atts) {
        extract( shortcode_atts( array(
            'content' => ''
        ), $atts ) );
        $this->data .= $content;
    }

    public function output_content() {
        echo $this->data;
    }
}
$wpa69616_plugin = new WPA69616_Plugin();