In your screenshot I see a Duplicati instance listening on poort 8300, so probably every users gets his own Duplicati instance (Default listening port is 8200).
My guess is that there is a shortcut to Duplicati.GUI.TrayIcon.exe in %PROGRAMDATA%\Microsoft\Windows\Start Menu\Programs\StartUp (the common Startup folder in the Start Menu).
Remove the shortcut from this location and install Duplicati as a Windows Service.