How Does Meta Event Deduplication Work?

Sending a purchase through both the browser and your server is a sound way to recover events the browser drops, but it gives Meta two reports of one action. Deduplication is how Meta spots the pair and keeps a single copy. Without it, Meta warns, your conversion rate, return on ad spend and attribution can all look higher than they really are, which can mislead budget decisions.

When do you need deduplication?

Only when you send the same event from both sources. If the browser sends Purchase and the server also sends Purchase, you must deduplicate. If each source sends different events, for example AddToCart from the browser and Purchase from the server, there is nothing to deduplicate.

Which keys does Meta match on?

Method	What must match	Limits
Event ID	event_name and event_id	Meta only deduplicates events received within 48 hours of the first event with that event_id
Fallback	event_name plus external_id or fbp	Works only for events sent first from the browser and the server event second; doesn’t work for a browser-only or server-only setup

Meta’s developer documentation says it decides whether events are identical based on their ID and name.

What happens when two events match?

Meta prefers the event it received first. The same rule covers three cases:

• repeated pixel events with the same event name and event ID;
• repeated server events with the same event_name and event_id;
• a pixel event and a server event that match on event name and event ID, or on event name and external_id or fbp.

How do you pass the event ID?

In the pixel, eventID is the fourth argument of the fbq track call. Meta’s developer example is:

fbq('track', 'Purchase', {value: 12, currency: 'USD'}, {eventID: 'EVENT_ID'});

On the server, you add the same value as event_id in the Conversions API event. Meta says many partner platforms, such as Shopify and WooCommerce, add these parameters for you. With a manual setup, you add them in your own code.

How do you check that deduplication works?

1. Open Events Manager, select the name and ID of your data, find the event and click View details.
2. Open the Event deduplication tab and read the deduplication keys table for the pixel and the Conversions API.
3. Check the overlap figure, the share of events with a given key received from both sources. Meta says higher is better.
4. Use the Test events tool to see which events Meta received, deduplicated or dropped.
5. Compare how event_name and event_id are formatted in both sources if counts still look doubled.

Note that the Events received figure on the Event overview tab adds both sources together before deduplication, so it will look larger than your real event count.

What usually breaks deduplication?

Meta lists three common causes: a missing or mismatched event_id, delays between browser or app events and server events, and wrong parameter names or values. Unexpected spikes or drops in event counts are the signal to check.

FAQ

Does deduplication matter if I only use the Conversions API?

It can. Meta deduplicates repeated server events that share the same event_name and event_id, so a retry with the same ID is counted once. The fbp or external_id fallback, though, does not deduplicate a server-only setup, so give every server event an ID.

How long after the first event can Meta still catch a duplicate?

Forty-eight hours. Meta’s developer documentation says events are only deduplicated if they arrive within 48 hours of when Meta receives the first event with a given event_id. A server event sent in a late batch after that window will not be deduplicated against the first one.

Do Shopify and WooCommerce handle deduplication for me?

Meta says many partner platforms, naming Shopify and WooCommerce, handle the deduplication parameters automatically. If you send events with your own code instead of a partner integration, you must add event_name and a unique event_id yourself, matching in browser and server events.