q303280 Article.
SYMPTOMS
When you use the Local Users and Groups snap-in to view
the members of a local group, the list may be empty or may
not display all the members of the group. If you try to re-
add a missing user account, you may receive the following
error:
The following error occurred while attempting to save
properties for group Administrators on computer computer
name: The specified account name is already a member of
the local group.
However, if you use the net user command on a missing
account, the account is returned and appears as a member
of the correct groups.
CAUSE
This problem may occur if a member of the local group has
a name that is the same as that of the domain. For
example, if the name of your domain is DOMAIN, and you add
a local user who is named LocalMachine\Domain, to a local
group, these symptoms may occur.


Seemingly this was fixed in SP3 and higher but I still
have the problem