The identification rule for these CI classes need to be reviewed and fixed. Even if serial number is not there, most of the hardware CI classes will ideally look for additional fields like 'Name' also based on the OOB identification rules.
As a temporary solution for CIs already populated, check if Serial Number and Name can be gathered and populated manually for the existing CIs for which these values are empty. This will help discovery to identify them correctly if the CI class has proper identifier entries. If some of those are already discovered and populated into a different record, may be you can use that information to populate missing attributes in the already existing correct CI records and then delete the invalid newly created CIs (assuming they are not referenced in any tasks yet).
However the proper long-term solution is setting the identifier entries correctly for each CI class. In the following article, I have explained about identification rules and the best practices to be considered while defining it:
https://servicenowguru.com/cmdb/servicenow-cmdb-identification-and-reconciliation-engine-ire/
