Переход с Django 1.11 на 2.1: есть повторяющиеся поля в 'fieldsets [2] [1]'

В настоящее время я переношу свое приложение Django 1.11 на Django 2.1. У меня есть собственная модель пользователя:

from authtools.models import AbstractEmailUser, UserManager
from django.db import models
from django.utils.translation import ugettext_lazy as _
from model_utils.fields import AutoLastModifiedField
from model_utils.models import SoftDeletableModel

from core.behaviors import UniversallyUniqueIdentifiable


class User(
    UniversallyUniqueIdentifiable,
    SoftDeletableModel,
    AbstractEmailUser
):
    """
    User should generally not be deleted, but rather is_removed should just
    be set to true. The delete() method is overwritten in the
    SoftDeletableModel.
    Also add a uuid field to avoid displaying the sequential primary key.
    """

    name = models.CharField(_('name'), max_length=255, blank=True)
    modified = AutoLastModifiedField(_('modified'))

    objects = UserManager()

И настраиваемый админ:

from authtools.admin import (BASE_FIELDS, SIMPLE_PERMISSION_FIELDS,
                             NamedUserAdmin)
from django.contrib import admin

from .models import User


def verified(obj):
    email = obj.emailaddress_set.filter(primary=True)
    if email.exists():
        return email[0].verified
    return False


verified.boolean = True


@admin.register(User)
class SoftDeletableNamedUserAdmin(NamedUserAdmin):
    """
    Overwrite the fields of the NamedUserAdmin to add is_removed.
    """

    date_hierarchy = "date_joined"
    list_display = (
        'email',
        'name',
        verified,
        'is_active',
        'is_removed',
    )
    search_fields = ["email", "name"]
    readonly_fields = ("date_joined", "modified")
    fieldsets = (
        BASE_FIELDS,
        SIMPLE_PERMISSION_FIELDS,
        ("Contact information", {
            "fields": (
                ("email", "name"),
            )
        }),
        ("Account information", {
            "fields": (
                "is_removed",
            ),
        }),
        ("Dates", {
            "fields": (
                ("date_joined", "modified",),
            ),
        })
    )
    list_filter = ('is_active', 'is_removed',)

Проблема в том, что при запуске сервера я получаю следующую ошибку:

django.core.management.base.SystemCheckError: SystemCheckError: System check identified some issues:

ERRORS:
<class 'accounts.admin.SoftDeletableNamedUserAdmin'>: (admin.E012) There are duplicate field(s) in 'fieldsets[2][1]'.
<class 'accounts.admin.SoftDeletableNamedUserAdmin'>: (admin.E012) There are duplicate field(s) in 'fieldsets[3][1]'.
<class 'accounts.admin.SoftDeletableNamedUserAdmin'>: (admin.E012) There are duplicate field(s) in 'fieldsets[4][1]'.

Я посмотрел на документация, чтобы увидеть, изменился ли способ определения этих кортежей, но я не заметил разницы. Что здесь не так?