With Sidecar now available for the iPad, Duet has adapted and introduced new functionality that's not available with Sidecar - namely using an additional Mac or PC as a secondary display.
As of today, Duet Air, the software that turns the iPad into a secondary display for a Mac, also works with a second Mac or PC.
That means if you have an extra MacBook or PC, you can use Duet Air to turn it into a wireless second display or a mirrored display for your main Mac. This is functionality that's not available with Apple's Sidecar implementation.
Duet Air also allows iOS devices, Macs, or PCs to remote desktop into other devices from anywhere in the world, and Duet says that the update works with "minimal latency."
Over the course of the next year, Duet has pledged to continue improving its algorithms and customizability, and there are a long list of new features in the works that will make Duet Air more compelling in 2020.