RMAN-05609: Must specify a username for target connection when using active duplicate

In 11gR2 it was possible to run an active duplication command for data guard and cloning of DB via duplication without explicitly specifying a username for the target instance.
But in 12c1 this would lead to RMAN-05609: Must specify a username for target connection when using active duplicate error.

$ rman target / auxiliary sys/ent12c1db@ent12c1stns

Recovery Manager: Release 12.1.0.1.0 - Production on Mon Aug 4 13:08:03 2014

Copyright (c) 1982, 2013, Oracle and/or its affiliates.  All rights reserved.

connected to target database: ENT12C1 (DBID=209099011)
connected to auxiliary database: ENT12C1S (not mounted)

RMAN> duplicate target database for standby from active database
2> spfile
3> parameter_value_convert 'ent12c1','ent12c1s','ENT12C1','ENT12C1S'
4> set db_unique_name='ent12c1s'
5> set db_file_name_convert='/data/oradata/ENT12C1','/opt/app/oracle/oradata/ENT12C1S'
6> set log_file_name_convert='/data/oradata/ENT12C1','/opt/app/oracle/oradata/ENT12C1S','/data/flash_recovery/ENT12C1','/opt/app/oracle/fast_recovery_area/ENT12C1S'
7> set control_files='/opt/app/oracle/oradata/ENT12C1S','/opt/app/oracle/fast_recovery_area/ENT12C1S'
8> set db_create_file_dest='/opt/app/oracle/oradata'
9> set db_recovery_file_dest='/opt/app/oracle/fast_recovery_area'
10> set log_archive_max_processes='10'
11> set fal_client='ENT12C1STNS'
12> set fal_server='ENT12C1TNS'
13> set log_archive_dest_2='service=ENT12C1TNS LGWR ASYNC NOAFFIRM max_failure=10 max_connections=5 reopen=180 valid_for=(online_logfiles,primary_role) db_unique_name=ent12c1'
14> set log_archive_dest_1='location=use_db_recovery_file_dest valid_for=(all_logfiles,all_roles) db_unique_name=ent12c1s';

Starting Duplicate Db at 04-AUG-14
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of Duplicate Db command at 08/04/2014 13:08:11
RMAN-05501: aborting duplication of target database
RMAN-05609: Must specify a username for target connection when using active duplicate

Solution is to include the username and password for the target instance as well.

$ rman target sys/ent12c1 auxiliary sys/ent12c1@ent12c1stns

This is different to how duplication was done on 11gR2 as such may require changes to duplication scripts when used with 12c.