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.

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

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