functions_user.php reports USERNAME_TAKEN_USERNAME if the new user being created matches an existing user name OR and existing group name.
When matching the group name, a group-specific error message should be reported, since this collision is likely the first introduction the administrator has to the idea that the names must be unique across both tables.
e.g. Define and return USERNAME_TAKEN_GROUPNAME as "The username you entered is already in use as a group name, please select an alternative." Otherwise the administrator is left to search their user list for this phantom "duplicate user name".
In acp_groups.php, under the premise that group names and user names ARE supposed to be unique across both tables, there is no check being performed to ensure uniqueness across both tables when creating a new group.
Meaning functions_user.php stops me from creating a user named "Banned" if a group named "Banned" already exists. But acp_groups.php DOES NOT stop me from creating that same condition, by creating a group named "Banned" if a user named "Banned" already exists.