That is the one of most popular Facebook CAPI errors. This error means that you are not sending some deduplication keys for your server events. Facebook uses these deduplication keys: event name, event ID, _fbp and external ID. 

Event ID parameters are causing this error > 90% of the time. Check if you are sending an event ID for Facebook Pixel and Facebook Conversion API. This event ID should be the same for the browser and server event. In this case, FB will see the same event name and event ID and start deduplication. Handling Duplicate Pixel and Conversions API Events

For example, for the ViewContent event, send the same event ID and event name from Facebook Pixel and FB CAPI.

To test event ID, open the Facebook event testing tool. If everything is correct, you should see something similar to the screenshot. That is how FB shows it recorded ViewContent events from both browser and server. These events had the same event ID. So they processed browser events and deduplicated server events.

After you have tested by the Facebook testing tool and It is showing deduplicate the same above video. You can ignore the problem because your pixel is handling duplicate data correctly the problem is just a nasty false positive. It will not impact your Facebook Ads

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Facebook Pixel App (The best solution for IOS 14 update): https://apps.shopify.com/yuri-facebook-multi-pixels

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