Duplicate Patients in FHIR Repository

Hoping someone can help,

I have the "Sample FHIR Production" installed ( as described in the documentation) with the following Components:

Service: HS.Gateway.ECR.SDA3XMLFileService

Processes: HS.FHIR.FromSDA.DTL.Transaction.Process

Operations: HS.FHIR.Repository.Operations

When a patient's data gets updated in my application a new SDA3 message is generated and processed by the FromSDA DTL. The Repository Stores that data as a New patient ( i.e. when I query the Repo they have different IDs "resource":{ "resourceType":"Patient", "id":"4", ) . How do I get the record in the Repository to update rather than store as a new Patient record?

Do I have to create a BPL which specifies that I want to do an "update" rather than a "create" ?

Perhaps it's just some field that I need to populate in my SDA3 ?

Any suggestions or code samples would be most helpful.

Thanks

Conor