According to documentation, Zend_Feed_Reader_Entry_Rss::getID() "Returns a unique ID for the current entry.": http://framework.zend.com/manual/en/…

In fact, the identifier is not necessarily unique. The method tries to obtain a uid from several of the item's fields:

/**
 * Get the entry ID
 *
 * @return string
 */
public function getId()
{
    if (array_key_exists('id', $this->_data)) {
        return $this->_data['id'];
    }

    $id = null;

    if ($this->getType() !== Zend_Feed_Reader::TYPE_RSS_10
        && $this->getType() !== Zend_Feed_Reader::TYPE_RSS_090
    ) {
        $id = $this->_xpath->evaluate('string('.$this->_xpathQueryRss.'/guid)');
    }

    if (!$id) {
        $id = $this->getExtension('DublinCore')->getId();
    }

    if (empty($id)) {
        $id = $this->getExtension('Atom')->getId();
    }

    if (!$id) {
        if ($this->getPermalink()) {
            $id = $this->getPermalink();
        } elseif ($this->getTitle()) {
            $id = $this->getTitle();
        } else {
            $id = null;
        }
    }

    $this->_data['id'] = $id;

    return $this->_data['id'];
}

For a RSS 2 feed without guid, getPermalink() (which uses the link element) is a fallback. As my test case feed shows, two items can have the same value for link. This causes 2 items to have the same return value for getId().