How to duplicate desktop in Linux with xrandr?

Currently, I'm using Deepin OS 15.4. By default Deepin sets the desktop to extended mode when a projector is connected, but what I want is to duplicate my screen. I can't find it in the Control Center configuration, but I stumbled upon in [this question](https://stackoverflow.com/questions/18736948/extended-desktop-in-linux) . The above extends the monitor with xrandr, How do I duplicate my desktop to the projector with xrandr?