In a report by Android Police, Google Assistant cam now be asked to initiate a video call with a contact on your device. Just ask Google Assistant to “Video call _____” followed by the contact’s name.
It worked perfectly with the Pixel 2 and also it immediately called the respective person. As reported by Android Police the feature is still rolling out so it may either use Hangouts to make the call or it might just open the Duo app and not do anything else.
According to The Verge, the command also works on Google Assistant for iOS. This is the best way to get in touch with others if you are a regular Duo user.