ORA-12528: TNS:listener: all appropriate instances are blocking new connections

If you try to duplicate database using rman and get following error:

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-00554: initialization of internal recovery manager package failed
RMAN-04006: error from auxiliary database: ORA-12528: TNS:listener: all appropriate instances are blocking new connections

Most probably your auxiliary database (in nomount state) is the only instance works on current ORACLE_HOME. Each instance is registered in listener by PMON process, which starts only when database is at least mounted. If there is nothing registered in listener before, instance in blocked.

You can do 2 things:
1. Modify listener.ora like this:

SID_LIST_LISTENER =
(SID_LIST =
(SID_DESC =
(GLOBAL_DBNAME = AUXDB)
(ORACLE_HOME = /app/oracle/product/10.2.0/)
(SID_NAME = AUXDB)
)
)
OR
2. Modify tnsnames.ora (10g only!) :

AUXDB =
(DESCRIPTION =
(ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))
(CONNECT_DATA =
(SERVER = DEDICATED)
(SERVICE_NAME = AUXDB)(UR=A)
)
)