If it’s not there (I have a feeling that AUR removed it, as it’s not in my Manjaro VM), you could try
/usr/bin/mono /opt/duplicati/Duplicati.GUI.TrayIcon.exe --webservice-port=8200
as you after shutting down any systemctl managed duplicati and any others that are running now.
When I do that, Manjaro puts the Duplicati icon (as above) in an icon bar at lower right of screen.
Icon gives me the menu shown on a right-click, or doing a left-click just opens the web UI directly.
Did you have such an icon before to start Duplicati GUI? How did you do it before? Asking again.