Event deduplication
Also known as: deduplication, event_id matching, double counting
Event deduplication is how an ad platform recognises that two reports of the same sale, one from the shopper's browser and one from the shop's server, are the same event and must be counted once. The platform matches them on a shared identifier the shop sends with both.
Why would the same sale be reported twice?
Because most shops run both a pixel and a server connection on purpose. The pixel reports what the browser saw; the server reports what the shop recorded. Sending both is what recovers the sales the browser loses, and event deduplication is the mechanism that stops the overlap becoming double counting.
Meta's own guidance is to run the two together rather than replace one with the other. The pair is only a problem when the shared identifier is missing or does not match.
What makes event deduplication fail?
A mismatched identifier, almost always. The browser and the server must send the same event ID, as the same string, with the same capitalisation. Generate it in two places by two different rules and the platform sees two unrelated sales, so one order is counted as two.
What does a broken deduplication cost?
Inflated numbers you then optimise against. A campaign credited with twice the sales it produced looks twice as profitable, so budget moves toward it and away from campaigns reporting honestly. The reporting error becomes a spending error, which is the expensive half.
Sources
- Meta deduplicates browser and server events that share the same `event_name` and `event_id`, and treats the identifier as case-sensitive.Meta for Developers
- wesaw derives the identifier from the order ID, so replaying the same purchase produces the same identifier and no second sale.wesaw, own test store
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