Created attachment 116886 [details]
duplicated notification example

SUMMARY
In those desktops with two o more monitors, and one panel on each, every notification popup is show in each monitor and one notification per monitor (i.e.: if you have three monitors, you'll see three popups on each monitor) by default, which seems a bug to me and has been already reported (https://bugs.kde.org/show_bug.cgi?id=393247)

I can disable completely notifications on additional monitor, which is not ideal, because I would like to see what's happening if I have my main monitor with some full screen application overlapping the panel. Or I can hide just system notifications or just transfer notifications on the additional monitors for not to show them on there, which in my opinion, isn't ideal either, because perhaps I would like to see for example file transfer notifications every now and then on that notification system tray.

STEPS TO REPRODUCE
1. Put a default panel in each monitor (two monitors minimum)
2. Transfer a set of files and wait to finish

OBSERVED RESULT
You'll see two notifications in each monitor (if you are on a two-monitor set up) (see attachment)

EXPECTED RESULT
At least, if you have notifications enabled on all system tray notification widget, you should only see one notification popup in each monitor, not as many popups as monitors

SOFTWARE/OS VERSIONS
KDE Plasma Version:  5.14.4
KDE Frameworks Version:  5.52.0
Qt Version: 5.11.2

ADDITIONAL INFORMATION
I think the ideal notification icon should be managed globally, not per monitor/panel. On notification settings, where now you let to choose the location of the notification popup, I would also add three options:
- show in all monitors
- show just in main monitor
- show the notification of the applications in the current monitor (just like the taskbar does) then, if for example, you want to work on the main monitor with yakuake at full screen, you can beforehand drag the dolphin window to your auxiliary monitor when copying those bunch of files and the notification will be shown on that monitor, even you start the file transfer on your main monitor.