What did I do? What is this?

Issue Summary

Hello,

I installed Drupal, installed OG and installed localizer.

After surfing around the Drupal.org page I wanted to try out uieforum. Installed it but did not get satisfied so I uninstalled it and switched back to the original Forum.

I created containers and Forums and now when I enter the forum it appears but right above it I can see around 10 times this message:

user warning: Not unique table/alias: 'tn' query: SELECT ncs.last_comment_timestamp, IF (ncs.last_comment_uid != 0, u2.name, ncs.last_comment_name) AS last_comment_name, ncs.last_comment_uid FROM node n INNER JOIN users u1 ON n.uid = u1.uid INNER JOIN term_node tn ON n.nid = tn.nid INNER JOIN node_comment_statistics ncs ON n.nid = ncs.nid INNER JOIN users u2 ON ncs.last_comment_uid=u2.uid INNER JOIN term_node tn ON tn.nid = n.nid WHERE (tn.tid = 1) AND ( n.status = 1 AND n.type='forum' AND tn.tid = 154 ) ORDER BY ncs.last_comment_timestamp DESC LIMIT 0, 1 in /var/www/xxxxxxxxxxxxxxxxxx/includes/database.mysql.inc on line 172.

Questions (since I want to learn about these things I haven't got only one)
1. What is this error
2. How did I cause it
3. What can I do about it?

Thank you for your support in advance.

By the way. A clean reinstall of Drupal is not possible since the site is already live and users have signed up.