Until there is an actual service installer, you need to run Duplicati.WindowsService.exe as an extra later step after an initial install (e.g. from .msi) that does NOT do the automatic or manual start (which is the tray icon).
By default the tray icon contains a Duplicati server, and it can also open the web browser to connect to that. When running as a Windows service, you probably want either no tray icon or run it with --no-hosted-server.