If you cloned a database and need to run both instances with the same db_name from one $ORACLE_HOME i.e.:
*.db_name=TESTDB
use the following parameters in your pfile or spfile:
*.lock_name_space=ANOTHERDB
*.sp_name=ANOTHERDB
Where ANOTHERDB is the name of the cloned sid.
Note: this has been tested with Oracle 10.1 only