session->cache do something just once

gistfile1.txt

public Your_extension_ext
{
    public function your_hook()
    {
        if ( ! isset($this->EE->session->cache['your_extension]))
        {
            //do your stuff
        }

        //set the cache, so next time your stuff won't run
        $this->EE->session->cache['your_extension'] = TRUE;
    }
}