According to a thread in the forums, Webmin's Read Mail module shows both user accounts when user@domain.tld account names are used. This is probably bug-like, and certainly confuses users. Would it be terrible to use the UID to strip identical users (preferring the @ user in the visible list)?
http://www.virtualmin.com/forums/virtualmin/duplicate-users-bug.html
Status:
Closed (fixed)