Leopard Permissions Unknown and Duplicate User

Odds are you updated to Leopard from another version rather than doing a clean install. The "unknown" user is most likely one that is no longer used such as the staff group.

In other words:
This is a bug in Leopard, only appearing on systems that are updated from Tiger. Running the 10.5.2 Combo Update should fix the permissions.
But … unfortunately one of the last updates (was it the Graphic-Update?) changed the current Build Number, so if you installed this updates, you won´t be able to run the 10.5.2 Combo Update (And you have to wait for 10.5.3)

There are "hacks", you will find on the internet, how to re-change the Build Number, but i will not recommend this -- of course it will work.
The other way is to set the permissions via Terminal.
I´m german, so i found a german link for you:
http://www.macmark.de/osxupdates.php#leopardgruppen

of course it seems to be nearly the same procedur, as described in the last post of this (english) thread:
http://www.macosxhints.com/article.php?story=200711291134001

Spazek

the unknown user is the legacy from Tiger group structure. In Tiger, the primary group for every user was a special private group with the same name as the username of the user. In leopard, by default the primary group is staff and leopard's finder doesn't understand old private groups and displays them as unknown. Finder used to crash because of this if you tried to change permissions. This was fixed in 10.5.2 so that it doesn't crash any more but the old private groups remain so you still see the unknown group in finder.

Personally, I found it quite annoying and changed all my users to leopard's group structure. various ways to do that but here is how I do it.

log out of the account you want to change and into a different admin account.

run the following in terminal:

*sudo dscl .delete /users/username*

where instead of username you put the short name of the user you want to change.
This deletes that user but not its home directory. Restart. go to system preferences->accounts and create a new user with the same shortname as the one you've just deleted. You'll see a popup saying that a home directory by that name already exists and asking if you want to use it. say "Yes". That's it, works like a charm. It also changes all the permissions on the home directory so no further work is needed.

To get rid of the unknown group on the external drive I run

*sudo chgrp -R staff /Volumes/"name-of-external"*

there might be some ACLs left which can be zapped by running

*sudo chmod -RN /Volumes/"name-of-external"*

This pretty much vanquished the remains of the Tiger group structure on my system and drives.

sorry, from the terminal commands you posted, I figured you were an experienced terminal user. If you are not, it's probably a good idea to leave things alone as nothing is actually broken at the moment.
Good luck!