How to Avoid Processing the Same Message Twice

Using a message broker?

Assume a message might arrive more than once.

One common approach is to make the consumer idempotent.

For example:

if (await processedMessages.ExistsAsync(message.Id))
{
    return;
}

await ProcessMessageAsync(message);

await processedMessages.AddAsync(message.Id);

How to Avoid Processing the Same Message Twice

An idempotent handler produces the same result even when the same message is delivered repeatedly.

Common techniques include:

Store processed message IDs
Use unique database constraints
Use idempotency keys
Make updates naturally idempotent
Execute state changes and deduplication atomically where needed

At-least-once delivery means duplicates are part of the design problem.

If the same message arrives twice, processing it twice should not create two different business outcomes.