Hey, i found an issue regarding this topic which explains the multiple usernames:
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=26564
on
http://groups.google.com/group/joomla-dev-general/browse_thread/thread/ca3ffee1bf076bd0
the solution was mentioned to add a DISTINCT in the sql command in
/libraries/joomla/html/html/list.php
But, well changing joomla core files is never good 😉
The other thing is, that it would be much better if the entrys in the drop down box would be the ‘username’, not the ‘name’.
The ‘username’ field is unique, the ‘name’ field isn’t