Meta CAPI Deduplication: How It Works and Why It Fails

Meta and CAPI · 4 min read · by Anna Viggeborn

When you send the same purchase from the browser pixel and from the Conversions API, Meta needs to recognise them as one event. That is deduplication, and it depends on the two fields you send with each copy.

What has to match

The event_id must be stable and tied to the order, not generated fresh in each system.

Why deduplication fails

• The server generates a random ID while the browser uses the order number.
• The event_id is sent only from one side.
• Event names differ in case or wording, for example Purchase and purchase.
• The server event arrives long after the browser event.
• A retry sends the server event twice with a new ID each time.

How to verify

In Events Manager, open the Purchase event and look at the connection overview. Meta reports how many events were received from each source and how many were deduplicated. If the deduplicated share is near zero while both sources are active, you are counting twice.

Inflated conversions look like a good week until the bank account disagrees.