Purged indicator-sound, removed from indicators. Installed xfce4-pulseaudio-plugin - added to panel. Restarted machine. Using volume keys results in 2 notifications.
Created attachment 6532 [details] screenshot
Additionally - when I login I get 1 notification show for no apparent reason.
This may be coming from xfce4-volumed running in background. If that is indeed the case, you can either switch xfce4-volumed off or disable notifications in the pulseaudio plugin properties dialog.
That appears to be the case for the 2 notifications. Still, shouldn't see one when just logging in I assume - certainly not touching volume at that point.
I see. This commit should fix it: 6431653 Do not show notifications at start up (bug #12313) Closing this report. Please reopen it if the fix doesn't work for you.