Most message brokers deliver at least once. How do you test that a consumer is idempotent?

Write tests that deliver the same message more than once. Then check that the effect happened only once.

The basic test: call the handler twice with the same payload and the same message id. Then check there is:

• one order row
• one charge on the stubbed payment gateway
• one email

The harder cases:

• The handler crashes after writing to the database but before it acks (confirms the message). The broker then delivers the message again.
• Two workers process the same message at the same time. Use Promise.all against a real database, so the unique constraint or lock is really tested.
• Messages arrive in the wrong order.

The code usually uses a "processed messages" table with a unique key, or a natural unique constraint. It writes this in the same transaction as the business change. The tests must prove this design holds. That is why they need the real database, not a mocked repository.