From the following tweet:
https://twitter.com/TotalSecurily/status/1027604783601995776

Chrome connects to FCM on start-up and will process all received notifications since the browser was last connected, which means a larger number of them might be posted at once.

It sounds (no pun intended) like the Action Center doesn't batch or serialize the sounds, so perhaps we should proactively mark notifications as silent if another had been shown in the past few seconds? The user's attention will already have been drawn at that point.