EM CC 12c installation blew up and need to re-create, since here are the instructions to remove an existing OEM CC (or OEM GC) repository of a failed installation using the following command:

$/sysman/admin/emdrep/bin/RepManager -dbUser sys -dbPassword -dbRole sysdba -reposName sysman -reposPassword -action dropall [-mwHome ] [-mwOraHome ]


For Oracle Enterprise Manager Database Control (single instance) repository use this command:
emca -deconfig dbcontrol db -repos drop -cluster -SYS_PWD sys1 -SYSMAN_PWD SYS_PASSWORD


HTH,
Pasi

Run the following command: emca -deconfig dbcontrol db -repos drop

C:\>emca -deconfig dbcontrol db -repos drop

STARTED EMCA at 3.2.2010 13:10:47
EM Configuration Assistant, Version 11.1.0.5.0 Production
Copyright (c) 2003, 2005, Oracle. All rights reserved.

Enter the following information:
Database SID: orcl
Listener port number: 1521
Password for SYS user:
Password for SYSMAN user:
Password for SYSMAN user:
Do you wish to continue? [yes(Y)/no(N)]: Y
3.2.2010 13:13:34 oracle.sysman.emcp.EMConfig perform
INFO: This operation is being logged at C:\app\oracle\cfgtoollogs\emca\orcl\em
_2010_02_03_13_10_47.log.
3.2.2010 13:13:45 oracle.sysman.emcp.EMDBPreConfig performDeconfiguration
WARNING: EM is not configured for this database. No EM-specific actions can be
erformed.
3.2.2010 13:13:50 oracle.sysman.emcp.EMReposConfig invoke
INFO: Dropping the EM repository (this may take a while) …
3.2.2010 13:34:40 oracle.sysman.emcp.EMReposConfig invoke
INFO: Repository successfully dropped
Enterprise Manager configuration completed successfully
FINISHED EMCA at 3.2.2010 13:34:40

The process may take a while.

After the drop you can reinstall the Enterprise Manager with: emca -config dbcontrol db -repos create

C:\>emca -config dbcontrol db -repos create

STARTED EMCA at 3.2.2010 13:37:18
EM Configuration Assistant, Version 11.1.0.5.0 Production
Copyright (c) 2003, 2005, Oracle. All rights reserved.

Enter the following information:
Database SID: orcl
Listener port number: 1521
Password for SYS user:
Password for DBSNMP user:
Password for SYSMAN user:
Password for SYSMAN user: Email address for notifications (optional):
Outgoing Mail (SMTP) server for notifications (optional):
—————————————————————–

You have specified the following settings

Database ORACLE_HOME ……………. C:\app\oracle\product\11.1.0\db_1

Local hostname ……………. IP_address
Listener port number ……………. 1521
Database SID ……………. orcl
Email address for notifications ……………
Outgoing Mail (SMTP) server for notifications ……………

—————————————————————–
Do you wish to continue? [yes(Y)/no(N)]: Y
3.2.2010 13:38:43 oracle.sysman.emcp.EMConfig perform
INFO: This operation is being logged at C:\app\oracle\cfgtoollogs\emca\orcl\emca
_2010_02_03_13_37_18.log.
3.2.2010 13:38:45 oracle.sysman.emcp.EMReposConfig createRepository
INFO: Creating the EM repository (this may take a while) …
3.2.2010 13:53:25 oracle.sysman.emcp.EMDBPostConfig performConfiguration
INFO: Database Control started successfully
3.2.2010 13:53:25 oracle.sysman.emcp.EMDBPostConfig performConfiguration
INFO: >>>>>>>>>>> The Database Control URL is https://IP_address:5500/em <<<<<<<
<<<<
3.2.2010 13:53:27 oracle.sysman.emcp.EMDBPostConfig invoke
WARNING:
************************ WARNING ************************

Management Repository has been placed in secure mode wherein Enterprise Manager
data will be encrypted. The encryption key has been placed in the file: C:\app\
oracle\product\11.1.0\db_1\10.8.0.171_orcl\sysman\config\emkey.ora. Please ens
ure this file is backed up as the encrypted data will become unusable if this fi
le is lost.

***********************************************************
Enterprise Manager configuration completed successfully
FINISHED EMCA at 3.2.2010 13:53:27

Great! Now you can try to log in: https://IP_address:5500/em