Hi -- if you have 2 identifier entries as described, then a CI with a unique (i.e. different) Serial number will create a new CI. The serial number id (100 in your example) will not find a match to an existing CI in the CMDB (per your example), hence a new Ci is created. The FQDN identifier won't matter (won't be evaluated by IRE in your example), because the 1st order identifier (100) didn't find a match for SN data in the payload. Since the payload has SN data but it didn't match, that should be enough for the IRE to determine this is a unique/different entity, and allow a new CI to be created.
If you have an old CI with an old SN, which has same FQDN as a new CI w/new, unique SN, then if appropriate consider retiring or archiving the CI. Worst case (and NOT a best practice) is to delete the old CI, but this has other possible ramifications so generally deleting is not a preferred option for dealing with old CIs. Another option that some customers use to dealing with old CI's is to alter/rename applicable identifier values, such as SN: 123 might be changed to: retired_123 so the IRE no longer matches old CI records. Food for thought anyway.
Hope this helps a bit?
