Creating new resources with multiple contained resources which lack id returns 500 Internal Server Error

[JeremiahSanders]

Describe the bug
When creating a new resource (Organization in executed tests) with multiple contained resources which lack a defined id, a 500 Internal Server Error is returned. This is triggered by an exception: ArgumentException: An item with the same key has already been added. Key: #. The exception appears to occur in Spark.Engine.Service.FhirServiceExtensions.IndexService.MakeContainedReferencesUnique().

To Reproduce

The following code, which uses the Spark reference server, shows the behavior.

Note that the resources in this example are minimal and not representative of a full model. However, they are sufficient to exhibit the error.

Uri baseUri = new Uri("https://spark.incendi.no/fhir/", UriKind.Absolute);
var httpClient = new HttpClient();
var settings = new FhirClientSettings
{
	VerifyFhirVersion = false,
	PreferredReturn = Prefer.ReturnRepresentation,
	PreferredFormat = ResourceFormat.Json
};
var fhirClient = new FhirClient(baseUri, httpClient, settings);
var organization = new Organization
{
	Name = "An Organization",
	Identifier = {
		new Identifier("http://a-fake-system", "a value")
	}
};

// Removal of EITHER contained resource (below) results in success. The error only occurs when multiple lack id.
organization.Contained.Add(new Endpoint
{
    Identifier = {
	        new Hl7.Fhir.Model.Identifier {
		    System = "http://not-a-real-system",
		    Value = "endpoint-1-identifier"
		}
	},
});
organization.Contained.Add(new Endpoint
{
	Identifier = {
		new Hl7.Fhir.Model.Identifier {
			System = "http://not-a-real-system",
			Value = "endpoint-2-identifier"
		}
	},
});

var createdOrg = await fhirClient.CreateAsync(organization);

Expected behavior
If the resource is valid, then a 201 Created should be returned. If the resource is invalid, then a 400-class response should be returned.

Spark version

• Version: 1.5.14