Last month, it started to look like Duo would add its own calls to your phone's call history. While the app asked users for permission to manage the call history, it didn't actually work yet. As part of the Duo 14 app update, this feature is now operational.
There's not much to this, but it's definitely a nice addition. When you open the app, Duo will ask you for permission to manage phone calls. Once you do that, any audio or video calls will show up in your call history. On the Google dialer app, video calls are marked with a small camera icon (there is no special icon for audio calls). On other dialers, Duo video/audio calls appear as normal calls.
There's not much else to this change, except that you'll need Duo v14 for it to work. If it hasn't rolled out to you on the Play Store yet, you can grab it from APKMirror.