Hi @矢野 尭生
Thank you for reaching us!
For your query, the User Principal Name (UPN) is a unique identifier for a user in Azure AD B2C. Each user in Azure AD B2C must have a unique UPN within the same tenant. However, the same UPN can be used to register consumer users across multiple Azure AD B2C tenants. since the uniqueness constraint applies only within a single tenant.
I hope this answer helps! If you have any further questions, please feel free to ask.
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