Identified the issue after so so long -

This is a well known issue in the community, it's just that fix was not so simple without breaking anything else.

https://stackoverflow.com/questions/66697332/firebase-web-push-notifications-is-triggered-twice-when-using-onbackgroundmessag#:~:text=3%20Comments&text=If%20you%20using%20Firebase%20Admin,send%20some%20data%20like%20below.&text=With%20FCM%2C%20you%20can%20send,property%20notification%20from%20the%20payload.&text=With%20that%2C%20FCM%20will%20not,doesn't%20has%20property%20notification%20.

https://firebase.google.com/docs/cloud-messaging/web/receive-messages

firebase/firebase-js-sdk#6670

So when the payload has both notification + data:

FCM's built-in push handler auto-displays the notification on the browser.
The Raven web SW's onBackgroundMessage fires and (based on the duplicate you're seeing in the screenshot) also calls self.registration.showNotification(...).
User sees two notifications — one from FCM's default handler, one from the custom SW.

But we can't simply remove notification as APNS and Android both have different configuration from web.

Requires a small but probably breaking change on client as well - The-Commit-Company/raven#2126