There are many alternatives to Duet Display for Mac if you are looking for a replacement. The best Mac alternative is Air Display. It's not free, so if you're looking for a free alternative, you could try Deskreen or Weylus. If that doesn't suit you, our users have ranked more than 10 alternatives to Duet Display and 11 are available for Mac so hopefully you can find a suitable replacement. Other interesting Mac alternatives to Duet Display are Splashtop Wired XDisplay (Paid), TwomonUSB (Paid), Astropad (Paid) and Yam Display (Paid).
Turn your iPad/iPhone or Android tablet or smartphone into a convenient side monitor for your Windows or Mac desktop. iDisplay is a universal app specially optimized for iPad, iPhone, iPod Touch, and Android devices screens.
iDisplay vs Duet Display opinions
Some users think iDisplay is a great alternative to Duet Display, some don't.
It works as long as the computer you are trying to extend and the Apple/Android device is connected to the same WiFi network. The downside is that it is not as smooth than duet display (but iDisplay is wireless nonetheless)