Ubuntu is showing multiple notification when volume is changed

I solved this by disabling xfce-volumed, Pulseaudio plugin was running along with it.

Open a Terminal window and type killall xfce4-volumed and try to change volume now. Only one notification will be shown.

To disable xfce4-volumed permanently open Session and Startup from settings manager. Go to Application Autostart tab. Look for XFCE Volume Daemon (pulseaudio) and untick it to disable.

Edit:

A more simple approach I found is to right-click on the volume icon in the panel and open properties. Uncheck the 'show notification when volume changes' box and log out/in.

This is happening because of inclusion of Unity related indicator in Xfce. Which is Indicator-Plugin. But if you remove it, you'll lose other indicators with it like messaging etc.

The Solution

Right click on an empty space of the Panel -> Panel -> Panel Properties
Go to Items Tab
Select Indicator Plugin
Click on the right side small wrench icon. Tooltip will say Edit the currently selected Item
On the new Indicators window, There is a list of indicators which are provided by this Indicator Plugin.
Place a tick mark for the Sound Menu entry. This will be hidden on next panel restart.
Close this Indicators window.
Press Alt-F2 to bring Run command dialog box.
Type xfce4-panel -r and Hit Enter.

Alternatively you can Logout/Login without performing step 8 and 9

The panel will be restarted and you'll see the volume icon is gone. Now, we need a volume icon for it.

Install xfce4-pulseaudio-plugin

sudo apt install xfce4-pulseaudio-plugin

Now again right click on the panel -> Panel -> Add New Items and Add PulseAudio Plugin
Move it to a suitable location in the panel.
Change volume and see only one notification (to rule them all!)