The Meta Pixel and Conversions API let you share web events with Meta (Facebook) Business Manager so that you can measure and optimize ad performance. The Meta Pixel tracks web events from a web browser, while the Conversions API tracks web events directly from your server.
If you connect website activity using both the Meta Pixel and Conversions API, Meta Business Manager may receive the same events from web browser and your server. If Meta Business Manager knows that the events are the same and therefore redundant, it can keep one and discard the rest. This is called deduplication.
For example, you send the same purchase events from the browser as you do from the server. This may help recover events that aren't received by the Meta Pixel. If your redundant browser and server events aren't deduplicated, you would see two purchases appear in your report (one sent from the browser and one sent from the server) when only a single purchase has actually occurred. Deduplication is necessary in this instance so that the same purchase doesn't show up twice in your reporting.
Once multiple events are received with the same event name and ID, Meta Business Manager employs a number of strategies to deduplicate between the events to maximize value for the advertiser. If your server and pixel events do not differ meaningfully in their content, it generally prefers the event that comes first.