[BUG] No 409 Conflict Returned On Posting Sub Model Reference Again (AAS REPO)

Describe the bug

In this method, no 409 conflict is returned (or an exception thrown) when adding a submodel reference if the reference already exists. However, the OpenAPI description specifies this error code. When removing, only the first occurrence of the reference is removed. So if the reference is added multiple times, we should probably remove all references.

Based on the OpenAPI description, I would assume that multiple references are not intended to be allowed, as the status code 409 suggests that duplicate entries should not be permitted.

To Reproduce

Deploy the reference multiple times
Get the shell and check for the submodels.

Expected behavior

In my opinion, the expected behavior would be to return 409

BaSyx (please complete the following information):

BaSyx Version [v2.0]
BaSyx SDK [Java]
AAS Version [DotAAS V3]

Screenshots

Used Asset Administration files (.aasx/.json/.xml)

Shell:

       {
                "modelType": "AssetAdministrationShell",
                "assetInformation": {
                    "assetKind": "Instance",
                    "globalAssetId": "https://acplt.org/Test_Asset"
                },
                "submodels": [
                    {
                        "keys": [
                            {
                                "type": "Submodel",
                                "value": "http://acplt.org/Submodels/Assets/TestAsset/BillOfMaterial"
                            }
                        ],
                        "type": "ExternalReference"
                    }
                ],
                "id": "https://acplt.org/Test_AssetAdministrationShell",
                "idShort": "TestAssetAdministrationShell"
            }

Additional submodel reference (already existing):

                 {
                        "keys": [
                            {
                                "type": "Submodel",
                                "value": "http://acplt.org/Submodels/Assets/TestAsset/BillOfMaterial"
                            }
                        ],
                        "type": "ExternalReference"
                    }

System (please complete the following information):

OS: Docker(Windows)
CPU architecture: x86

eclipse-basyx / basyx-java-server-sdk

[BUG] No 409 Conflict Returned On Posting Sub Model Reference Again (AAS REPO) #394