Yeah we've had the same issue, just finished blogging about it: http://www.kintek.com.au/blog/zend-db-and-joins/
You need to name each field uniquely, it's a known bug in Zend: http://framework.zend.com/issues/browse/ZF-6421?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel