How to make Duplicati start automatically with Ubuntu?

The icon comes from the tray-icon (Duplicati.GUI.TrayIcon.exe), what you enabled (with @prez’s help) was the SERVICE.

The service is actually the important part where Duplicati does it’s thing - it will even run scheduled backups even if the tray icon isn’t showing or a user isn’t logged into the computer.

If you want the tray icon to start when you log into the the computer, you’ll need to add the tray-icon to your session / startup info.

Note that different distributions may work differently, but this should give you a starting point…