is identities case sensitive?

Yes, identity values in the email namespace are case-sensitive in Adobe Experience Platform. This means that "User@example.com" and "user@example.com" are recognized as distinct identities and both will appear as separate nodes in the identity graph if ingested as such. This behavior is consistent across most string-based identity namespaces in AEP, including email.

Here’s the relevant information from Experience League documentation and best practices:

• "Only string fields can be marked as identities. And be aware that the identities are case-sensitive, and it’s really important."
  (Best Practices and Insights for Modeling XDM Schemas)

• "The value for the selected Person ID is considered to be case sensitive. For example, abc123 and ABC123 are two different values."
  (Create or edit a connection)

This is why you might see the same email address twice in the identity graph if the case differs, such as "John.Doe@example.com" and "john.doe@example.com".

Recommendation:
If you want to prevent duplicates, ensure that your source systems and data processing pipelines normalize email address case (commonly to lowercase) before ingestion.