Event deduplication is the matching process an ad platform uses to recognize that two incoming events describe the same action. The result: a conversion is counted once, even if it was reported by both the browser and a server.
How does event deduplication work?
Modern setups combine Client-Side Tracking and Server-Side Tracking. The Meta Pixel sends an event from the browser, and the Conversions API sends the same event from a server. To let the platform merge them, both carry a shared event ID, typically a random string generated per action.
At Meta, the event name and event ID must match. If both events arrive within a certain time window, Meta keeps one and discards the other. The TikTok Events API follows a similar principle with an event ID.
Why does event deduplication matter?
Without deduplication you count conversions twice, which leads to:
- reports showing too many leads or purchases, making ROAS and CPA look better than they are
- the algorithm optimizing on skewed data
- budget decisions based on wrong numbers
At the same time, sending events twice is intentional. If one of them fails to arrive, for instance because of an ad blocker, the other still gets through. Deduplication is what makes that redundancy usable.
Example
A visitor submits a contact form. The pixel sends a Lead event with ID lead-8f3a, and the server sends Lead with lead-8f3a as well. Meta sees the matching ID and records one lead, not two. If one of the events lacks the ID, you end up with two leads.
Event deduplication with LeadSignal
LeadSignal automatically assigns an event ID to every event and sends it with both the browser event and the server event, wherever a pixel and a server API run in parallel. There is nothing to configure and no code to change. For why combining browser and server makes sense, read the guide on server-side tracking without GTM.