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

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

@@ -1437,6 +1437,7 @@ private void ProcessMember(
                     return null;
                 }
 
+                HashSet<string>? memberInitializerNames = null;
                 List<PropertyInitializerGenerationSpec>? propertyInitializers = null;
                 int paramCount = constructorParameters?.Length ?? 0;
 
@@ -1450,6 +1451,18 @@ private void ProcessMember(
 
                     if ((property.IsRequired && !constructorSetsRequiredMembers) || property.IsInitOnlySetter)
                     {
+                        if (!(memberInitializerNames ??= new()).Add(property.MemberName))
+                        {
+                            // We've already added another member initializer with the same name to our spec list.
+                            // Duplicates can occur here because the provided list of properties includes shadowed members.
+                            // This is because we generate metadata for *all* members, including shadowed or ignored ones,
+                            // since we need to re-run the deduplication algorithm taking run-time configuration into account.
+                            // This is a simple deduplication that keeps the first result for each member name --
+                            // this should be fine since the properties are listed from most derived to least derived order,
+                            // so the second instance of a member name is always shadowed by the first.
+                            continue;
+                        }
+
                         ParameterGenerationSpec? matchingConstructorParameter = GetMatchingConstructorParameter(property, constructorParameters);
 
                         if (property.IsRequired || matchingConstructorParameter is null)

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

@@ -664,6 +664,52 @@ public class ClassWithCustomRequiredPropertyName
             public required int PropertyWithInitOnlySetter { get; init; }
         }
 
+        [Fact]
+        public async Task DerivedClassWithRequiredProperty()
+        {
+            var options = Serializer.CreateOptions(includeFields: true);
+            var value = new DerivedClassWithRequiredInitOnlyProperty { MyInt = 42, MyBool = true, MyString = "42", MyProp = 42.0M, MyLong = 4242, MyMember1 = 1, MyMember2 = 2, MyField = "42" };
+            string json = await Serializer.SerializeWrapper(value, options);
+            Assert.Equal("""{"MyInt":42,"MyBool":true,"MyString":"42","MyProp":42.0,"MyLong":4242,"MyMember1":1,"MyMember2":2,"MyField":"42"}""", json);
+
+            value = await Serializer.DeserializeWrapper<DerivedClassWithRequiredInitOnlyProperty>(json, options);
+            Assert.Equal(42, value.MyInt);
+            Assert.True(value.MyBool);
+            Assert.Equal("42", value.MyString);
+            Assert.Equal(42.0M, value.MyProp);
+            Assert.Equal(4242, value.MyLong);
+            Assert.Equal(1, value.MyMember1);
+            Assert.Equal(2, value.MyMember2);
+            Assert.Equal("42", value.MyField);
+        }
+
+        public class BaseClassWithInitOnlyProperty
+        {
+            public int MyInt { get; init; }
+            public bool MyBool { get; init; }
+            public string MyString { get; set; }
+            public string MyProp { get; init; }
+
+            public string MyMember1;
+            public string MyMember2 { get; init; }
+
+            public string MyField;
+        }
+
+        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 new required decimal MyProp { get; init; }
+            public required long MyLong { get; init; }
+
+            public new required int MyMember1 { get; init; }
+            public new required int MyMember2;
+
+            public new required string MyField;
+        }
+
         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
         {
         }

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Unit.Tests/JsonSourceGeneratorTests.cs

@@ -768,5 +768,32 @@ internal partial class JsonContext : JsonSerializerContext
             Compilation compilation = CompilationHelper.CreateCompilation(source);
             CompilationHelper.RunJsonSourceGenerator(compilation);
         }
+
+#if ROSLYN4_4_OR_GREATER && NETCOREAPP
+        [Fact]
+        public void ShadowedMemberInitializers()
+        {
+            string source = """
+                using System.Text.Json.Serialization;
+
+                public record Base
+                {
+                    public string Value { get; init; }
+                }
+                public record Derived : Base
+                {
+                    public new string Value { get; init; }
+                }
+
+                [JsonSerializable(typeof(Derived))]
+                public partial class MyContext : JsonSerializerContext
+                {
+                }
+                """;
+
+            Compilation compilation = CompilationHelper.CreateCompilation(source, parseOptions: CompilationHelper.CreateParseOptions(LanguageVersion.CSharp11));
+            CompilationHelper.RunJsonSourceGenerator(compilation);
+        }
+#endif
     }
 }