Cause: The external name specified for
the user being created or altered already exists for another user.
Action: External names must be unique
among users. Specify another.

Relared Errors:
ORA-28027:
privileged database links may be used by global users
ORA-28028:
could not authenticate remote server
ORA-28029:
could not authorize remote server for user string
ORA-28030:
Server encountered problems accessing LDAP directory service
ORA-28031:
maximum of string enabled roles exceeded
ORA-28035:
Cannot Get Session Key for Authentication
ORA-28037:
Cannot Get Session Key for RACF Authentication
ORA-28038:
disallow O2LOGON
ORA-28039:
cannot validate Kerberos service ticket
ORA-28040: No
matching authentication protocol