Note: This article has the reference to the issue with the fix available:
It is not possible to create a user for the mailbox:
PLESK_ERROR: The user with the username email@example.com already exists and owns another mailbox.
firstname.lastname@example.org not displayed in Plesk.
Database inconsistency caused by bug PPPM-8022.
Connect to the server via SSH
Delete the user via the command-line interface:
# plesk bin user -d email@example.com