In the event that a user was duplicated the standard practice is to deactivate that user so that they no longer show in people selectors. In very rare cases, trying to access that user once they've been deactivated returns a no access message.

Currently, as this bug is so rare, it's quite difficult to diagnose. If this user is still showing in user lists, the only known workaround currently is to append the real user's last name with something to identify it as the actual record such as "(real)".