STJ: Avoid duplicate initialization of required or init-only properties

src/libraries/System.Text.Json/gen/JsonSourceGenerator.Parser.cs

@@ -891,7 +891,9 @@ private List<PropertyGenerationSpec> ParsePropertyGenerationSpecs(
                             // property is static or an indexer
                             propertyInfo.IsStatic || propertyInfo.Parameters.Length > 0 ||
                             // It is overridden by a derived property
-                            PropertyIsOverriddenAndIgnored(propertyInfo, state.IgnoredMembers))
+                            PropertyIsOverriddenAndIgnored(propertyInfo, state.IgnoredMembers) ||
+                            // It is shadowed by a derived property
+                            PropertyIsShadowed(propertyInfo, state.AddedProperties))
                         {
                             continue;
                         }
@@ -980,6 +982,13 @@ bool PropertyIsOverriddenAndIgnored(IPropertySymbol property, Dictionary<string,
                         ignoredMember.IsVirtual() &&
                         SymbolEqualityComparer.Default.Equals(property.Type, ignoredMember.GetMemberType());
                 }
+
+                bool PropertyIsShadowed(IPropertySymbol propertyInfo, Dictionary<string, (PropertyGenerationSpec, ISymbol, int index)> addedProperties)
+                {
+                    return addedProperties.TryGetValue(propertyInfo.Name, out (PropertyGenerationSpec propertySpec, ISymbol symbol, int index) propertyItem) &&
+                        propertyInfo.IsOverriddenOrShadowedBy(propertyItem.symbol) &&
+                        propertyItem.propertySpec.DefaultIgnoreCondition != JsonIgnoreCondition.Always;
+                }
             }
 
             private ref struct PropertyHierarchyResolutionState(SourceGenerationOptionsSpec? options)

src/libraries/System.Text.Json/tests/Common/RequiredKeywordTests.cs

@@ -664,6 +664,35 @@ public class ClassWithCustomRequiredPropertyName
             public required int PropertyWithInitOnlySetter { get; init; }
         }
 
+        [Fact]
+        public async Task DerivedClassWithRequiredProperty()
+        {
+            var value = new DerivedClassWithRequiredInitOnlyProperty { MyInt = 42, MyBool = true, MyLong = 4242 };
+            string json = await Serializer.SerializeWrapper(value);
+            Assert.Equal("""{"MyInt":42,"MyBool":true,"MyString":null,"MyLong":4242}""", json);
+
+            value = await Serializer.DeserializeWrapper<DerivedClassWithRequiredInitOnlyProperty>(json);
+            Assert.Equal(42, value.MyInt);
+            Assert.True(value.MyBool);
+            Assert.Null(value.MyString);
+            Assert.Equal(4242, value.MyLong);
+        }
+
+        public class BaseClassWithInitOnlyProperty
+        {
+            public int MyInt { get; init; }
+            public bool MyBool { get; init; }
+            public string MyString { get; set; }
+        }
+
+        public class DerivedClassWithRequiredInitOnlyProperty : BaseClassWithInitOnlyProperty
+        {
+            public new required int MyInt { get; init; }
+            public new required bool MyBool { get; set; }
+            public new string MyString { get; init; }
+            public required long MyLong { get; init; }
+        }
+
         public static IEnumerable<object[]> InheritedPersonWithRequiredMembersSetsRequiredMembersWorksAsExpectedSources()
         {
             yield return new object[]

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/RequiredKeywordTests.cs

@@ -32,6 +32,7 @@ public RequiredKeywordTests_SourceGen()
         [JsonSerializable(typeof(ClassWithRequiredExtensionDataProperty))]
         [JsonSerializable(typeof(ClassWithRequiredKeywordAndJsonRequiredCustomAttribute))]
         [JsonSerializable(typeof(ClassWithCustomRequiredPropertyName))]
+        [JsonSerializable(typeof(DerivedClassWithRequiredInitOnlyProperty))]
         internal sealed partial class RequiredKeywordTestsContext : JsonSerializerContext
         {
         }