Registration results in Error Message: Service exception: [301] unique constraint violated

When resistering to the event https://sitregparticipant-a5a504e08.dispatcher.hana.ondemand.com/#/Events/93 a POST request is sent to https://sitregparticipant-a5a504e08.dispatcher.hana.ondemand.com/destinations/HANAMDC/com/sap/sapmentors/sitreg/odataparticipant/service.xsodata/$batch with this payload:

{
  "ID": "6149",
  "EventID": "93",
  "FirstName": "John",
  "LastName": "Doe",
  "EMail": "john@example.com",
  "RSVP": "Y",
  "History.CreatedBy": "",
  "History.ChangedBy": "",
  "__metadata": {
    "type": "com.sap.sapmentors.sitreg.odataparticipant.service.ParticipantType"
  },
  "Receipt": "Y",
  "PreEveningEvent": "Y",
  "PostEveningEvent": "Y",
  "BioURL": "",
  "Twitter": "",
  "MobilePhone": "",
  "RelationToSAP": "P",
  "ReceiptCompany": "John Doe Inc.",
  "ReceiptAddress": "123 Main St., Anytown, USA"
}

The following error is returned:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<error xmlns="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata">
  <code />
  <message xml:lang="en-US">Service exception: [301] unique constraint violated</message>
</error>

sapmentors / SITreg

Registration results in Error Message: Service exception: [301] unique constraint violated #56