diff --git a/django/contrib/admin/checks.py b/django/contrib/admin/checks.py
index 855e916eaa..4238297b22 100644
--- a/django/contrib/admin/checks.py
+++ b/django/contrib/admin/checks.py
@@ -3,7 +3,7 @@ from __future__ import unicode_literals
 
 from itertools import chain
 
-from django.contrib.admin.utils import get_fields_from_path, NotRelationField
+from django.contrib.admin.utils import get_fields_from_path, NotRelationField, flatten
 from django.core import checks
 from django.db import models
 from django.db.models.fields import FieldDoesNotExist
@@ -84,7 +84,8 @@ class BaseModelAdminChecks(object):
                     id='admin.E005',
                 )
             ]
-        elif len(cls.fields) != len(set(cls.fields)):
+        fields = flatten(cls.fields)
+        if len(fields) != len(set(fields)):
             return [
                 checks.Error(
                     'There are duplicate field(s) in "fields".',
@@ -93,11 +94,11 @@ class BaseModelAdminChecks(object):
                     id='admin.E006',
                 )
             ]
-        else:
-            return list(chain(*[
+
+        return list(chain(*[
                 self._check_field_spec(cls, model, field_name, 'fields')
                 for field_name in cls.fields
-            ]))
+        ]))
 
     def _check_fieldsets(self, cls, model):
         """ Check that fieldsets is properly formatted and doesn't contain
@@ -132,7 +133,9 @@ class BaseModelAdminChecks(object):
                     id='admin.E011',
                 )
             ]
-        elif len(fieldset[1]['fields']) != len(set(fieldset[1]['fields'])):
+
+        fields = flatten(fieldset[1]['fields'])
+        if len(fields) != len(set(fields)):
             return [
                 checks.Error(
                     'There are duplicate field(s) in "%s[1]".' % label,
@@ -141,11 +144,10 @@ class BaseModelAdminChecks(object):
                     id='admin.E012',
                 )
             ]
-        else:
-            return list(chain(*[
-                self._check_field_spec(cls, model, fields, '%s[1][\'fields\']' % label)
-                for fields in fieldset[1]['fields']
-            ]))
+        return list(chain(*[
+            self._check_field_spec(cls, model, fields, '%s[1][\'fields\']' % label)
+            for fields in fieldset[1]['fields']
+        ]))
 
     def _check_field_spec(self, cls, model, fields, label):
         """ `fields` should be an item of `fields` or an item of
diff --git a/django/contrib/admin/utils.py b/django/contrib/admin/utils.py
index a2f2e9fa7b..b52300756b 100644
--- a/django/contrib/admin/utils.py
+++ b/django/contrib/admin/utils.py
@@ -83,15 +83,25 @@ def unquote(s):
     return "".join(res)
 
 
+def flatten(fields):
+    """Returns a list which is a single level of flattening of the
+    original list."""
+    flat = []
+    for field in fields:
+        if isinstance(field, (list, tuple)):
+            flat.extend(field)
+        else:
+            flat.append(field)
+    return flat
+
+
 def flatten_fieldsets(fieldsets):
     """Returns a list of field names from an admin fieldsets structure."""
     field_names = []
     for name, opts in fieldsets:
-        for field in opts['fields']:
-            if isinstance(field, (list, tuple)):
-                field_names.extend(field)
-            else:
-                field_names.append(field)
+        field_names.extend(
+            flatten(opts['fields'])
+        )
     return field_names
 
 
diff --git a/tests/admin_checks/tests.py b/tests/admin_checks/tests.py
index ba9faea03f..109161ebba 100644
--- a/tests/admin_checks/tests.py
+++ b/tests/admin_checks/tests.py
@@ -463,3 +463,37 @@ class SystemChecksTestCase(TestCase):
                 )
             ]
             self.assertEqual(errors, expected)
+
+    def test_check_sublists_for_duplicates(self):
+        class MyModelAdmin(admin.ModelAdmin):
+            fields = ['state', ['state']]
+
+        errors = MyModelAdmin.check(model=Song)
+        expected = [
+            checks.Error(
+                'There are duplicate field(s) in "fields".',
+                hint=None,
+                obj=MyModelAdmin,
+                id='admin.E006'
+            )
+        ]
+        self.assertEqual(errors, expected)
+
+    def test_check_fieldset_sublists_for_duplicates(self):
+        class MyModelAdmin(admin.ModelAdmin):
+            fieldsets = [
+                (None, {
+                    'fields': ['title', 'album', ('title', 'album')]
+                }),
+            ]
+
+        errors = MyModelAdmin.check(model=Song)
+        expected = [
+            checks.Error(
+                'There are duplicate field(s) in "fieldsets[0][1]".',
+                hint=None,
+                obj=MyModelAdmin,
+                id='admin.E012'
+            )
+        ]
+        self.assertEqual(errors, expected)
diff --git a/tests/admin_util/tests.py b/tests/admin_util/tests.py
index 4cb2d843fb..20980efffa 100644
--- a/tests/admin_util/tests.py
+++ b/tests/admin_util/tests.py
@@ -5,8 +5,8 @@ from datetime import datetime
 from django.conf import settings
 from django.contrib import admin
 from django.contrib.admin import helpers
-from django.contrib.admin.utils import (display_for_field, flatten_fieldsets,
-    label_for_field, lookup_field, NestedObjects)
+from django.contrib.admin.utils import (display_for_field, flatten,
+    flatten_fieldsets, label_for_field, lookup_field, NestedObjects)
 from django.contrib.admin.views.main import EMPTY_CHANGELIST_VALUE
 from django.contrib.sites.models import Site
 from django.db import models, DEFAULT_DB_ALIAS
@@ -323,6 +323,17 @@ class UtilTests(SimpleTestCase):
         self.assertHTMLEqual(helpers.AdminField(form, 'cb', is_first=False).label_tag(),
                              '<label for="id_cb" class="vCheckboxLabel required inline">&amp;cb</label>')
 
+    def test_flatten(self):
+        flat_all = ['url', 'title', 'content', 'sites']
+        inputs = (
+            ((), []),
+            (('url', 'title', ('content', 'sites')), flat_all),
+            (('url', 'title', 'content', 'sites'), flat_all),
+            ((('url', 'title'), ('content', 'sites')), flat_all)
+        )
+        for orig, expected in inputs:
+            self.assertEqual(flatten(orig), expected)
+
     def test_flatten_fieldsets(self):
         """
         Regression test for #18051