When I create a new user via Virtualmin two users are created: one following Server Templates’s scheme (firstname.lastname@example.org) and one where the @ is replaced by a - (user-domain.tld). Both users have the same User ID, group, shell and home directory.
Where/how can I get rid of the 2nd creation?
That’s correct… that’s because Postfix doesn’t like using users with ‘@’ symbols in them. Creating a user with a ‘-’ in the name is a way around that restriction.
There’s some additional details here in the section at the end labeled “What’s the deal with @ in mailbox usernames?”:
The only way around that issue would be to not use usernames with an ‘@’ in their name.
… and sorry for reading the FAQ not carefully enough.