The JsonSerializer.Serialize result has duplicate fields.

[yuxingz]

Description

Duplicate fields occur when serialized using JSON Polymorphic.
The type field was duplicated in the reproduction step.

Reproduction Steps

[JsonPolymorphic(TypeDiscriminatorPropertyName = "type")]
[JsonDerivedType(typeof(Teatcher), 0)]
[JsonDerivedType(typeof(Student), 1)]
public interface IPerson
{
    int Type { get; }
    string Name { get; }
}

public class Teatcher : IPerson
{
    public int Type => 0;
    public string Name => "Teatcher";
}
public class Student : IPerson
{
    public int Type => 1;
    public string Name => "Student";
}
public class Model
{
    public IPerson? Person { get; set; }
}

[TestClass]
public class UnitTestClass
{
    [TestMethod]
    public void MyTestMethod()
    {
        try
        {
            var options = new JsonSerializerOptions
            {
                PropertyNamingPolicy = JsonNamingPolicy.CamelCase
            };

            var t = new Teatcher();
            var model = new Model { Person = t };
            var json = JsonSerializer.Serialize(model, options);
            //{"person":{"type":0,"type":0,"name":"Teatcher"}}
            Assert.IsNotNull(json);
        }
        catch (Exception ex)
        {
            throw;
        }
    }
}

Expected behavior

var json = JsonSerializer.Serialize(model, options);
//{"person":{"type":0,"name":"Teatcher"}}
//Instead of duplicated fields

Actual behavior

Has duplicated fields
{"person":{"type":0,"type":0,"name":"Teatcher"}}

Regression?

No response

Known Workarounds

No response

Configuration

dotnet --info
.NET SDK:
 Version:           9.0.100
 Commit:            59db016f11
 Workload version:  9.0.100-manifests.c6f19616
 MSBuild version:   17.12.7+5b8665660

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.19044
 OS Platform: Windows
 RID:         win-x64
 Base Path:   C:\Program Files\dotnet\sdk\9.0.100\

Host:
  Version:      9.0.0
  Architecture: x64
  Commit:       9d5a6a9aa4

.NET SDKs installed:
  8.0.100 [C:\Program Files\dotnet\sdk]
  9.0.100 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 8.0.11 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 9.0.0 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 8.0.11 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 9.0.0 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 6.0.36 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 8.0.11 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 9.0.0 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
  x86   [C:\Program Files (x86)\dotnet]
    registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
  Not set

global.json file:
  Not found

Learn more:
  https://aka.ms/dotnet/info

Download .NET:
  https://aka.ms/dotnet/download

project.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

</Project>

Other information

No response

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[eiriktsarpalis]

Duplicate of #72170.