Why are some events deleted by Facebook (event deduplication)?

The pixel and Conversions API let you share standard and custom events with Facebook so you can measure and optimize ad performance. As a reminder, the pixel lets you share web events from a web browser, while the Conversions API lets you share web events directly from your server.

If you connect the activity of your website using both the pixel and Conversions API, Facebook may receive the same events from the browser and the server. If such redundancy is detected, then Facebook can choose to keep one of the two events and delete the other. This is called deduplication.

For example, if the same purchase event is sent by both the pixel and your server (through the Conversions API), then, if nothing is done, two purchases (one corresponding to the purchase event sent by the browser and another corresponding to the purchase event sent by the server) will appear in your report, while only one purchase actually occurred. This is why Facebook deduplication is necessary so that the same purchase does not appear twice in your reports.

In order for events to be deduplicated, our module adds two event parameters so that Facebook can determine if the events are identical: the event name (e.g. Purchase or AddToCart) and the event ID.
When Facebook receives multiple events with the same name and ID, strategies to deduplicate the events (i.e., keep only one of the two events) are applied. When a server event and a browser event occur at approximately the same time (15 seconds apart), Facebook favors the browser event (i.e. the one sent by the pixel).

You can check which events are deduplicated using the test events tool in Meta Events Manager.

Click here to learn more about Facebook's event deduplication.