Event ID

An event ID is a shared code attached to a conversion so the browser copy and the server copy can be matched as one.

When you send the same event from both the pixel and the Conversions API, each copy carries the identical event ID. The platform uses that code to recognize the two messages are the same real action and collapse them into one, which is how deduplication actually works under the hood.

Picture a checkout that generates event ID order-9f2a. The browser fires Purchase with order-9f2a and your server sends Purchase with order-9f2a. Meta lines up the IDs, sees a match, and counts one sale. If the server instead sent order-9f2b, the match fails and you get two.

The ID has to be genuinely the same string on both sides, not just similar. Generate it fresh in each place, or format it differently, and the platform treats one purchase as two, inflating everything downstream.

Same code both sides, or the count splits in two.