I assume you used Duplicati.WindowsService.exe to register Duplicati as a Windows service.
If you want to access the Duplicati Web interface over the network, unregister the service using Duplicati.WindowsService.exe uninstall and register the service again, adding --webservice-interface=any, for example:
Duplicati.WindowsService.exe install --webservice-port=8200 --webservice-interface=any.
If you have a software firewall, like Windows Firewall, be sure to open the appropriate port for incoming network traffic.