Fix JsonSchemaFlattenAttribute duplicate fields with multiple inheritance levels

src/NJsonSchema.Tests/Generation/FlattenInheritanceHierarchyTests.cs

@@ -171,6 +171,61 @@ public async Task When_JsonSchemaFlattenAttribute_is_used_on_class_then_inherite
             Assert.True(schema.Definitions["B"].Properties.ContainsKey("Ccc"));
         }
 
+        [JsonSchemaFlatten]
+        public class FlattenedChild : MiddleClass
+        {
+            public string Name { get; set; }
+        }
+
+        public class MiddleClass : BaseClass
+        {
+            public string FirstName { get; set; }
+        }
+
+        public class BaseClass
+        {
+            public string LastName { get; set; }
+        }
+
+        [Fact]
+        public async Task When_JsonSchemaFlattenAttribute_with_multiple_inheritance_levels_then_no_duplicate_properties()
+        {
+            // Arrange
+            var settings = new NewtonsoftJsonSchemaGeneratorSettings();
+
+            // Act
+            var schema = NewtonsoftJsonSchemaGenerator.FromType<FlattenedChild>(settings);
+            var data = schema.ToJson();
+
+            // Assert
+            Assert.True(schema.Properties.ContainsKey("Name"));
+            Assert.True(schema.Properties.ContainsKey("FirstName"));
+            Assert.True(schema.Properties.ContainsKey("LastName"));
+            Assert.Empty(schema.AllOf);
+            Assert.False(schema.Definitions.ContainsKey("MiddleClass"));
+            Assert.False(schema.Definitions.ContainsKey("BaseClass"));
+        }
+
+        [Fact]
+        public async Task When_FlattenInheritanceHierarchy_setting_with_multiple_levels_then_no_duplicate_properties()
+        {
+            // Arrange
+            var settings = new NewtonsoftJsonSchemaGeneratorSettings
+            {
+                FlattenInheritanceHierarchy = true
+            };
+
+            // Act - Note: FlattenedChild also has the attribute, but we test with the setting too
+            var schema = NewtonsoftJsonSchemaGenerator.FromType<FlattenedChild>(settings);
+            var data = schema.ToJson();
+
+            // Assert
+            Assert.True(schema.Properties.ContainsKey("Name"));
+            Assert.True(schema.Properties.ContainsKey("FirstName"));
+            Assert.True(schema.Properties.ContainsKey("LastName"));
+            Assert.Empty(schema.AllOf);
+        }
+
         [Fact]
         public async Task When_class_inherited_and_json_flattened_then_ignore_base_property_with_same_name()
         {

src/NJsonSchema/Generation/JsonSchemaGenerator.cs

@@ -986,7 +986,7 @@ private void AddKnownType(Type type, JsonSchemaResolver schemaResolver)
                         if (!typeDescription.IsDictionary && !type.Type.IsArray)
                         {
                             Settings.ReflectionService.GenerateProperties(schema, baseType, Settings, this, schemaResolver);
-                            var actualSchema = GenerateInheritance(baseType, schema, schemaResolver);
+                            var actualSchema = FlattenAncestorProperties(baseType, schema, schemaResolver);
 
                             GenerateInheritanceDiscriminator(baseType, schema, actualSchema ?? schema);
                         }
@@ -1058,6 +1058,27 @@ private void AddKnownType(Type type, JsonSchemaResolver schemaResolver)
             return null;
         }
 
+        private JsonSchema? FlattenAncestorProperties(ContextualType type, JsonSchema schema, JsonSchemaResolver schemaResolver)
+        {
+            var baseType = type.BaseType;
+            if (baseType != null && baseType.Type != typeof(object) && baseType.Type != typeof(ValueType))
+            {
+                if (baseType.GetContextOrTypeAttributes(false).FirstAssignableToTypeNameOrDefault("JsonSchemaIgnoreAttribute", TypeNameStyle.Name) == null &&
+                    baseType.GetContextOrTypeAttributes(false).FirstAssignableToTypeNameOrDefault("SwaggerIgnoreAttribute", TypeNameStyle.Name) == null &&
+                    Settings.ExcludedTypeNames?.Contains(baseType.Type.FullName) != true)
+                {
+                    var typeDescription = Settings.ReflectionService.GetDescription(baseType, Settings);
+                    if (!typeDescription.IsDictionary && !type.Type.IsArray)
+                    {
+                        Settings.ReflectionService.GenerateProperties(schema, baseType, Settings, this, schemaResolver);
+                        return FlattenAncestorProperties(baseType, schema, schemaResolver);
+                    }
+                }
+            }
+
+            return null;
+        }
+
         private void GenerateInheritanceDiscriminator(Type type, JsonSchema schema, JsonSchema typeSchema)
         {
             if (!Settings.GetActualFlattenInheritanceHierarchy(type))