This issue has been fixed in RSA Key Manager Server 2.1.2.0.1 and RSA Key Manager Server 2.2.  Either apply 2.1.2.0.1 fix to your RKM Server 2.1.2, or upgrade to RKM Server 2.2.

If you can't upgrade, follow those instruction:

First, make sure you only have one certificate with the serial number, run:
            select * from auth_certificate where serialnumber = '<serial number>'

Then when you are certain, run:
            delete from auth_certificate where serialnumber = '<serial number>'
            commit;