Recently, I faced with the same issue for all events (Purchase Event from the Server are not Deduplicated in Facebook Event Manager).

like

Addtocart Event from the Server are not Deduplicated

InitiateCheckout Event from the Server are not Deduplicated

Purchase Event from the Server are not Deduplicated

ViewContent Event from the Server are not Deduplicated

Then I found a blog related to this issue and its solution, offering actionable steps you can try:

https://orichi.info/2025/03/17/event-from-the-server-are-not-deduplicated/