Looks like the service identifier (SID) name clash. Since the listerner would use the SID to connect to the instance, you would have to fix that one.

One way to fix this is wipe out all the instance file and try again.
.../admin/SIDs
.../oradata/SIDs
$ORACLE_HOME/dbs/xxxSID.*

Then also do a search from the oratab file name and check to see if it exists.

One other question is did you get to execute to root.sh script that would create the oratab files after installation? If not you might want to execute that before you proceed with the next database creation using dbassist.

Sam