ORA-15020: discovered duplicate ASM disk

I was installing Grid infrastructure for a standalone server.

By default, the diskstring was ORCL:* but I changed it to /dev/oracleasm/disks. However, ths disks were visible on both diskstrings. After the change, I faced the following error message at the end of installation:

INFO: Command /u01/app/grid/product/11.2.0/grid/bin/asmca -silent -oui_internal -configureASM -diskString /dev/oracleasm/disks -diskGroupName CRS -diskList /dev/oracleasm/disks/CRSVOL1,/dev/oracleasm/disks/CRSVOL2,/dev/oracleasm/disks/CRSVOL3 -redundancy EXTERNAL
INFO: ... GenericInternalPlugIn.handleProcess() entered.
INFO: ... GenericInternalPlugIn: getting configAssistantParmas.
INFO: ... GenericInternalPlugIn: checking secretArguments.
INFO: ... GenericInternalPlugIn: starting read loop.
INFO: Read: SYS_PASSWORD_PROMPT
INFO: Processing: SYS_PASSWORD_PROMPT for argument tag -sysAsmPassword
INFO: Read: ASMSNMP_PASSWORD_PROMPT
INFO: Processing: ASMSNMP_PASSWORD_PROMPT for argument tag -asmMonitorPassword
INFO: End of argument passing to stdin
INFO: Read:
INFO: Read: Configuring ASM failed with the following message:
INFO: Read: One or more diskgroup(s) creation failed as below:
INFO: Read: DiskGroup CRS creation failed with the following message:
INFO: Read: ORA-15018: diskgroup cannot be created
INFO: Read: ORA-15020: discovered duplicate ASM disk "CRS_0002"
INFO: Read:
INFO: Read:
INFO: Completed Plugin named: Automatic Storage Management Configuration Assistant

Solution:

I changed the diskstring to ORCL as shown below and ran the below command as grid user

 /u01/app/grid/product/11.2.0/grid/bin/asmca -silent -oui_internal -configureASM -diskString ORCL:* -diskGroupName CRS -diskList ORCL:CRSVOL1,ORCL:CRSVOL2,ORCL:CRSVOL3 -redundancy EXTERNAL

If the ASM configuration assitant fails during the installation, then you can run the command manually after changing the diskstring and skip the step on the GUI installer after the command completes successfully.

In my case, the corrected command ran successfully.