MirrorDisplays error every 30 seconds in system.log I'm seeing an error every 30 seconds in the system log, using the Console app, complaining about MirrorDisplays service only running for 0 seconds.
Nov  5 22:45:22 computer com.apple.xpc.launchd[1] (com.apple.preference.displays.MirrorDisplays): Service only ran for 0 seconds. Pushing respawn out by 10 seconds.
Nov  5 22:45:52 --- last message repeated 2 times ---
Nov  5 22:45:52 computer com.apple.xpc.launchd[1] (com.apple.preference.displays.MirrorDisplays): Service only ran for 0 seconds. Pushing respawn out by 10 seconds.
Nov  5 22:46:22 --- last message repeated 2 times ---
Nov  5 22:46:22 computer com.apple.xpc.launchd[1] (com.apple.preference.displays.MirrorDisplays): Service only ran for 0 seconds. Pushing respawn out by 10 seconds.
Nov  5 22:46:52 --- last message repeated 2 times ---
Nov  5 22:46:52 computer com.apple.xpc.launchd[1] (com.apple.preference.displays.MirrorDisplays): Service only ran for 0 seconds. Pushing respawn out by 10 seconds.

It doesn't matter if I have no external display or use an external display at the same time.  How do I turn off this repeated error in the logs?
I had the same log complaints. 
They are the result of macOS detecting the secondary HDMI display and some code quality issue. 

Uncheck "System Preference > Display > Show mirroring options in the menu bar when available." After reboot the log output will be gone.