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I do the following:  (this example is in phpsh)

php> set_include_path('/usr/local/apache2/htdocs/lib/zfdev/trunk/library');
php> require_once 'Zend/Session.php';
php> Zend_Session::start();
php> for ( $i = 0; $i < 10; $i++ ) {
 ... Zend_Session::regenerateId(true);
 ... var_dump(Zend_Session::getId());
 ... }

And the result is like this:
string(26) "ko1c1649uconma5kal2ugvtir5"
string(26) "0itbujiagvpcvd9nl9nc0gc0r6"
string(26) "0itbujiagvpcvd9nl9nc0gc0r6"
string(26) "6r3cjcaqobqptpf0veacjr4nj6"
string(26) "6r3cjcaqobqptpf0veacjr4nj6"
string(26) "rio861fofl0m8et9on8dtupah6"
string(26) "rio861fofl0m8et9on8dtupah6"
string(26) "jrpiitcceq55ik49d11e8nbre6"
string(26) "jrpiitcceq55ik49d11e8nbre6"
string(26) "d442id0mi40jr56cugc3er9rr3"

Pairs of session IDs? Is that normal?