Can you help understand the business use case here? Below is a snip from MS documentation...
"Azure AD Connect sync server. An on-premises computer that runs the Azure AD Connect sync service. This service synchronizes information held in the on-premises Active Directory to Azure AD. For example, if you provision or deprovision groups and users on-premises, these changes propagate to Azure AD."
I am trying to understand, under what circumstances will you have the need to explicitly check the uniqueness in On-Prem AD as well as Azure AD
External Ref: https://learn.microsoft.com/en-us/azure/architecture/reference-architectures/identity/azure-ad