How to Use a Single Cookie in Magento? Easy Steps

You can be able to fix this by reversing the order of the session_start() call and the statement that sets the cookie in Mage_Core_Model_Session_Abstract_Varien::start(..).

Those two lines now look like this:

$cookie->set(session_name(), $this->getSessionId());
session_start();

It now only creates one single cookie and it does not seem to have any side-effects.

The other cookie was not created in Zend_Session, instead both of them came from Mage_Core_Model_Session_Abstract_Varien::start(..)