Each message that is dispatched from an Endpoint has a unique identity. NServiceBus generates and stores this value as a header on the message named NServiceBus..
Many features take advantage of message identity. For example, the Outbox uses message identity to deduplicate messages and recoverability uses message identity to keep track of how many times the endpoint has tried to process a message.
Specifying message identity
Message identity can be explicitly specified, overriding the default identity provided by NServiceBus.
It is important that the strategy used to generate message identities results in globally unique identifiers. If two messages ever have the same identity then some features will treat them as the same message. This will cause errors which are difficult to diagnose.
Specify message identity using the SendOptions class.
var options = new SendOptions();
options.SetMessageId(messageId);
await handlerContext.Send(new SomeMessage(), options)
.ConfigureAwait(false);