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.

1 files changed, 15 insertions, 5 deletions

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