Event deduplication
Matching the browser event and the server event for the same action so Meta counts it once.
Once you run both the pixel and the Conversions API, the same form submission can arrive at Meta twice: once from the visitor’s browser, once from your server. Deduplication is how Meta is told these are one event. Both copies carry the same event ID and the same event name, and Meta keeps the first and discards the second.
Why it matters for HVAC
A contractor running both sources without deduplication sees their lead count inflate, sometimes close to double. That is bad in an obvious way and worse in a subtle one. The obvious problem is that the reports are wrong. The subtle problem is that cost per lead looks about half what it really is, which makes a losing campaign look like a winner and gets it scaled.
This is the most common way a technically correct Conversions API setup produces worse decisions than no Conversions API at all.
How it is measured
Events Manager reports deduplication directly, showing how many events were received and how many were discarded as duplicates. If server events are arriving and nothing is being deduplicated, the IDs are not matching. Comparing a day’s reported conversions against the same day’s CRM enquiries catches it from the other direction.
A worked example
A contractor turns on server events and their monthly lead count jumps from 80 to 151 with no change in spend. Cost per lead appears to halve. Nothing improved: 71 of those leads are the same people counted twice, because the server events were sent without an event ID. The account then scales a campaign on the strength of a number that was never real.
These figures are invented to show the arithmetic. They describe no real business.
What to ask
Ask how pixel and server events are deduplicated. The answer should mention an event ID generated once and sent with both copies. An answer that does not mention an ID at all is a warning.
Where this is demonstrated
The Tracking and the Conversions API pillar scores every listed agency on what they document here, and gives you the questions that tell you which one you are being sold.