> ## Documentation Index
> Fetch the complete documentation index at: https://upstash-vector.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Deduplication

Messages can be deduplicated to prevent duplicate messages from being sent. When
a duplicate message is detected, it is accepted by QStash but not enqueued. This
can be useful when the connection between your service and QStash fails, and you
never receive the acknowledgement. You can simply retry publishing and can be
sure that the message will enqueued only once.

In case a message is a duplicate, we will accept the request and return the
messageID of the existing message. The only difference will be the response
status code. We'll send HTTP `202 Accepted` code in case of a duplicate message.

## Deduplication ID

To deduplicate a message, you can send the `Upstash-Deduplication-Id` header
when publishing the message.

```
curl -XPOST \
    -H 'Authorization: Bearer XXX' \
    -H "Content-type: application/json" \
    -H "Upstash-Deduplication-Id: abcdef" \
    -d '{ "hello": "world" }' \
    'https://qstash.upstash.io/v2/publish/...'
```

## Content Based Deduplication

If you want to deduplicate messages automatically, you can set the
`Upstash-Content-Based-Deduplication` header to `true`.

```
curl -XPOST \
    -H 'Authorization: Bearer XXX' \
    -H "Content-type: application/json" \
    -H "Upstash-Content-Based-Deduplication: true" \
    -d '{ "hello": "world" }' \
    'https://qstash.upstash.io/v2/publish/...'
```

Content based deduplication creates a unique deduplication ID for the message
based on the following fields:

* **Destination**: The topic or endpoint you are publishing the message to.

* **Body**: The body of the message.

* **Header**: This includes the `Content-Type` header and all headers, that you
  forwarded with the `Upstash-Forward-` prefix. See
  [custom HTTP headers section](/qstash/howto/publishing#sending-custom-http-headers).