Can't add certificate with same subject and different issuer

[vokac]

It is not possible to add certificate with same subject and just different issuer. There are a lot of accounts in legacy VOMS where synchronization script fails to create compatible https://github.com/indigo-iam/voms-importer/issues/8 list of certificates in IAM, because there are a lot of same subject entries that differs just with issuer, e.g. /DC=ch/DC=cern/CN=CERN Grid Certification Authority vs. /DC=ch/DC=cern/CN=CERN Trusted Certification Authority.

Why IAM doesn't support certificate with same subject? Why attempt to add same certificate subject + different issuer succeeds with HTTP 204 but there is no real change in the IAM?