Sharing some thoughts, as well as saved the potentially fixable problem to the end for some reason...
I have started experimenting a bit with Zorin/KDE/GS Connect, and find it really cool! Though, its also one of those install you gotta give some thought into when setting up, to avoid a lot of spam/duplicate notifications.
First epiphany was when I enabled the setting "Notification sync" (or was it "Share notifications"?). At least there was a setting that was off by default, that ensured to sync cleared notifications both ways. Huge improvement, and should be default on, imho.
Second, I have slack, email and calendars on both devices. Syncing notifications on apps that are installed on both sides, will most likely result in a lot of "duplicates". To me, a recommendation to sync only notifications on services that are unique to phone seems sensible.
Third, Outlook on Android seems to have some old bug that stops it from clearing mobile notification if mail are read on pc/server. This is really annoying, but only solution at the moment is to use different app on mobile I guess.
Fourth, UI is not consistent using multiple monitors. For some reason (Ubuntu related?) notifications are located in the date meny and not the system menu on additional monitors. Any tip to adjust this maybe?
Primary monitor placement is in System menu:
Secondary and Third monitor placement is in Date menu:
