Fixed #22018 -- Fixed checks for ModelAdmin.fields not handling sub-lists.

Flatten a level of sublists before checking for duplicate fields.

When given sublists such as:

```python

class FooAdmin(admin.ModelAdmin):
    fields = ('one', ('one', 'two'))
```

The previous code did not correctly detect the duplicated 'one' field.

Thanks to jwa for the report.

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