My new phone was getting Facebook push and email notifications, (i.e.) double notifications from facebook, (i.e) I was being notified that there was something on my Facebook page even after I'd already gone on Facebook and looked at it. So I looked at my previous phone which didn't get these double notifications and saw the difference in their settings.
Here's the procedure (on Facebook version 287.0.0.50.119, on Android version 10 (formerly called Android Q)) to stop the push or double notifications:
Tap:
Facebook app, hamburger (three upper right corner lines), scroll down to and tap Settings and Privacy, Settings, scroll down to Notifications and tap Notifications Settings,
Now you have two options:
1. under Where You Receive Notifications, tap Push and tap Mute Push Notifications, after which the longest option to stop push notifications is 8 hours
or
2. tap Other Notifications and under Where you receive these notifications, move the slider across from Push to the left to turn it off.
You have the option to turn off email notifications there too, but I left email notifications on.