The Oracle system identifier ” already exists, specify another SID ” error occurs during database creation using dbca utility. It is for almost all the versions like Oracle 11g, 12c, 19c ..etc.
ROOT CAUSE: You might have created the database with same name and didn’t removed it properly using GUI. Some traces or entries are still there.
Solution:
Open /etc/oratab file using vi editor and remove the line containing SID that is causing an error message. Another place to look would be $ORACLE_BASE/oraInventory/ContentsXML/inventory.xml – this file could also contain an entry with the offending SID. If .xml file is not present then no problem you can proceed with the installation now.