How to Prevent Duplicate Message Processing with Inbox Pattern

The Challenge

Duplicate message processing is something every event-driven system eventually faces. With at-least-once delivery, retries and redeliveries are expected. The challenge is making sure processing the same message twice does not create side effects.

The Inbox Pattern

I've been looking into the Inbox Pattern as a consumer-side solution:

• Track processed messages.
• Keep message tracking and business changes in the same transaction scope.
• Perform the idempotency check per consumer.

One approach is using a MassTransit pipeline filter so the idempotency logic stays outside the consumers.

How do you usually handle this? Do you use Inbox Pattern, custom middleware, database constraints, or something else?