Fixed #29322 -- Made admin check all ModelAdmin fieldsets for duplicates.

tests/modeladmin/test_checks.py

@@ -166,6 +166,19 @@ class FieldsetsCheckTests(CheckTestCase):
             'admin.E012'
         )
 
+    def test_duplicate_fields_in_fieldsets(self):
+        class TestModelAdmin(ModelAdmin):
+            fieldsets = [
+                (None, {'fields': ['name']}),
+                (None, {'fields': ['name']}),
+            ]
+
+        self.assertIsInvalid(
+            TestModelAdmin, ValidationTestModel,
+            "There are duplicate field(s) in 'fieldsets[1][1]'.",
+            'admin.E012'
+        )
+
     def test_fieldsets_with_custom_form_validation(self):
         class BandAdmin(ModelAdmin):
             fieldsets = (('Band', {'fields': ('name',)}),)