Thank you for the pointer to the Microsoft article. However, after walking through clearing keychains of "exchange," "adal," and "office" following the article's steps, the end-user continues to be challenged repeatedly.
When launching Outlook, the end-user is asked to enter a CAC PIN three times. For each new message, the end-user is asked to enter a PIN again, once for each new message.
We now have two end-users who both have dual persona CACs, both of whom are using Centrify Express 5.4.2 and Outlook 15.40 who report the same issue. There is one end-user who uses a similar dual persona CAC who reports that the issue was resolved when the Mac was swtiched to Thursby's PKard for Mac as middleware, although I have not been able to reach out to verify this solution.