So if you have two admin classes that inherit from PageAdmin, the extra fields from the first one get appended to PageAdmin.unknown_fields, and the second one blows up when the auto-generated ModelForm tries to reference those fields.

module/page/modeladmins.py
33:    unknown_fields = ['template_key', 'parent', 'override_url', 'redirect_to']
44:            'fields': unknown_fields,
87:                self.unknown_fields.append(f.name)