Fix - Ensure `WagtailAdminFormPageForm.clean` works for any related name

Dynamically detect the `related_name` of any form fields attached to the page so that validation is actually run for custom related_names.

This also will apply validation even if there are multiple `AbstractFormField` subclasses related to the page.

Add unit test to ensure that duplicate clean name validation correctly runs on FormPage models with a custom related name.

Originally from #10375

wagtail/contrib/forms/forms.py

@@ -173,17 +173,34 @@ class SelectDateForm(django.forms.Form):
 
 class WagtailAdminFormPageForm(WagtailAdminPageForm):
     def clean(self):
-        cleaned_data = super().clean()
-        related_name = "form_fields"
+        """
+        Dynamically detect all related AbstractFormField subclasses to ensure
+        validation is applied regardless of the related_name or if there are multiple
+        AbstractFormField subclasses related to this page.
+        """
+
+        from .models import AbstractFormField
+
+        cleaned_data = super().clean()
+
+        form_fields_related_names = [
+            related_object.related_name
+            for related_object in self.instance._meta.related_objects
+            if issubclass(related_object.related_model, AbstractFormField)
+        ]
+
+        for related_name in form_fields_related_names:
+            if related_name not in self.formsets:
+                continue
 
-        # Check for duplicate form fields by comparing their internal clean_names
-        if "form_fields" in self.formsets:
             forms = self.formsets[related_name].forms
             for form in forms:
                 form.is_valid()
 
-            # Use existing clean_name or generate for new fields.
-            # clean_name is set in FormField.save
+            # Use existing clean_name or generate for new fields,
+            # raise an error if there are duplicate resolved clean names.
+            # Note: `clean_name` is set in `FormField.save`.
+
             clean_names = [
                 f.instance.clean_name or f.instance.get_field_clean_name()
                 for f in forms

wagtail/contrib/forms/tests/test_views.py

@@ -1973,6 +1973,52 @@ class TestDuplicateFormFieldLabels(WagtailTestUtils, TestCase):
             text="There is another field with the label Test field, please change one of them.",
         )
 
+    def test_adding_duplicate_form_labels_with_custom_related_name(self):
+        """
+        Ensure duplicate field names are checked, even if a custom related_name is used.
+        ``FormPageWithCustomSubmission`` uses the related_name `custom_form_fields`.
+        """
+
+        post_data = {
+            "title": "Drink selection",
+            "content": "Some content",
+            "slug": "drink-selection",
+            "custom_form_fields-TOTAL_FORMS": "3",
+            "custom_form_fields-INITIAL_FORMS": "3",
+            "custom_form_fields-MIN_NUM_FORMS": "0",
+            "custom_form_fields-MAX_NUM_FORMS": "1000",
+            # Duplicate field labels
+            "custom_form_fields-0-id": "",
+            "custom_form_fields-0-label": "chocolate",
+            "custom_form_fields-0-field_type": "singleline",
+            "custom_form_fields-1-id": "",
+            "custom_form_fields-1-label": "chocolate",
+            "custom_form_fields-1-field_type": "singleline",
+            # Unique field label
+            "custom_form_fields-2-id": "",
+            "custom_form_fields-2-label": "coffee",
+            "custom_form_fields-2-field_type": "singleline",
+        }
+
+        response = self.client.post(
+            reverse(
+                "wagtailadmin_pages:add",
+                args=(
+                    "tests",
+                    "formpagewithcustomsubmission",
+                    self.root_page.id,
+                ),
+            ),
+            post_data,
+        )
+
+        self.assertEqual(response.status_code, 200)
+
+        self.assertContains(
+            response,
+            text="There is another field with the label chocolate, please change one of them.",
+        )
+
 
 class TestPreview(WagtailTestUtils, TestCase):
     post_data = {