MantisHub doesn't allow two users to have the same user names, real names, or username of one being the same as the real name of the other. Hence, in an attempt to create or update a user with a non-unique name, the administrator will get an error message.
It is important to note that by default the Manage Users page doesn't show disabled users, so if the list is short, the easy approach would be to click "Show Disabled" and then apply filter. That will show the conflict users whether such user is enabled or disabled.
If the list of users is long, then the best approach is to use the "Search" field in the bottom of the Manage Users list. This field supports searching by username, real name or email address. So it is easy to try each one and see the record with the conflict.