If you re-use Active Directory (AD) UPNs (or email address) for user accounts in the Project Web App, the user's historic data may be incorrect.
Active Directory (AD)UPNs (or email address) cannot be reused for user accounts by Project Web App. The AD group user may appear to synchronize properly with Project Web App (PWA), but if the UPN already exists for a resource in PWA you see unexpected behavior.
For example: John Doe is an employee at Contoso, his UPN or email is JDoe@contoso.com. His account is added to an AD group that is used to synchronize users to PWA security groups. John Doe then leaves the company, his account is deleted from the AD group and on the next synchronization with PWA his account is marked as Inactive. This is by design to protect any history and reporting within PWA. Later, a new employee by the name of Jane Doe joins the company. At Contoso they have a policy to reuse UPNs when available. Jane is given the UPN or email address of JDoe@contoso.com. Jane is added to an AD group that synchronizes with PWA security groups. When the sync occurs, PWA is matching the UPN only. PWA sees that UPN already assigned to an Inactive resource and links up the new AD user with the old PWA user which is not the desired behavior. Historic data will be incorrect.
The recommendation from Microsoft Project is to not reuse UPNs within Active Directory. UPNs need to be unique within in PWA in order to function as designed.
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