New json deserializer silently accepts duplicate choice type properties of differing types

[almostchristian]

Describe the bug
The new System.Text.Json based deserializer silently accepts duplicate choice types if they are of different types. The old deserializer fails with an exception. From what I observed, it will use the first property encountered and ignore the rest.

To Reproduce
Reproduction code:

var scenario = """
    {
      "resourceType": "Patient",
      "deceasedBoolean": true,
      "deceasedDateTime": "2022"
    }
    """;

var jsonSerializerOptions = new JsonSerializerOptions().ForFhir();

// successfully parses with new deserializer
var patient = JsonSerializer.Deserialize<Patient>(scenario, jsonSerializerOptions);

Console.WriteLine("New deserializer: " + patient.ToJson());

// fails with old jsonParser
try
{
    patient = new FhirJsonParser().Parse<Patient>(scenario);
}
catch (Exception ex)
{
    Console.WriteLine(ex.ToString());
}

Output:

New deserializer: {"resourceType":"Patient","deceasedBoolean":"2022"}
System.FormatException: While building a POCO: Element 'deceased' must not repeat (at Patient.deceasedDateTime[0])
   at Hl7.Fhir.Serialization.ComplexTypeReader.RaiseFormatError(String message, String location)
   at Hl7.Fhir.Serialization.ComplexTypeReader.read(ClassMapping mapping, IEnumerable`1 members, Base existing)
   at Hl7.Fhir.Serialization.ComplexTypeReader.Deserialize(ClassMapping mapping, Base existing)
   at Hl7.Fhir.Serialization.PocoBuilder.<>c__DisplayClass9_0.<BuildFrom>g__build|0()
   at Hl7.Fhir.Serialization.PocoBuilder.BuildFrom(ITypedElement source)
   at Hl7.Fhir.Serialization.PocoBuilder.BuildFrom(ISourceNode source, ClassMapping mapping)
   at Hl7.Fhir.Serialization.PocoBuilder.BuildFrom(ISourceNode source, Type dataType)
   at Hl7.Fhir.ElementModel.PocoBuilderExtensions.ToPoco(ISourceNode source, ModelInspector inspector, Type pocoType, PocoBuilderSettings settings)
   at Hl7.Fhir.Serialization.BaseFhirParser.Parse(ISourceNode node, Type type)
   at Hl7.Fhir.Serialization.FhirJsonParser.Parse(String json, Type dataType)
   at Hl7.Fhir.Serialization.FhirJsonParser.Parse[T](String json)
   at Program.<Main>$(String[] args) in D:\repos\ConsoleApp1\Program.cs:line 26

Expected behavior
An exception should be thrown similar to the old deserializer.

Version used:

• Version: 5.5.0