Duet Display install caused reboot to hang on High Sierra
I've just encountered the same problem on my 15" 2016 MacBook Pro  running macOS 10.14.2 Mojave. While I expected it to take some time starting up it never came back to the login screen and here's how I solved the problem:


Reboot into single user mode by pressing the cmd + s keys on
startup. Note this not safe mode, it's single user mode.
Once in single user mode type mount -uw / so that you get write access to the drive.
Navigate to the kexts directory with cd /Library/Extentions.
Now delete the Duet Display kexts with sudo rm -Rf DuetDisplay.kext, it's a directory thus the -Rf.
Next clear caches with sudo rm -Rf /System/Library/Caches/*.
Finally restart with shutdown -r now.


Hopefully that helps anyone else with this issue. I'm so disappointed in Duet Display and asking for my money back. It's cost me more in loss of productivity fixing this mess than I hoped to gain using their App.