Configuration file as to set mirror display as a default for external monitor through command line

You could use xrandr

At first run "xranr" and look for "… connected" lines to find out the names of your displays. Then run
xrandr --output VGA1 --auto --same-as LVDS1
To make the display VGA1 a mirror or LVDS1.
"--same-as" is the parameter for setting up mirroring.
If you want flipping or scaling, see the other parameters of xranr.

Hi Thomas thanks for input, but my concern is to give this option on default, at the time of installation of OS, so that any user when connects frist time a external monitor, he gets mirror screen as a default. Currently it gets a external screen when it conects any monitor.

Well, this is a question of personal preference and the default should not be changed without considering all pros and cons properly.
If you have a good and balanced argumentation that conludes the change should be made, you can create a bug report for this.

But I will tell you right away: If your points are not rock solid and argumentation very good, you will get nowhere with it.

The question seems to be to change a local site default, not the upstream software's default. So simply the option to customize. He will know his site or organization's priorities. I need both lightdm and all users to default to mirror, because all users get c onfused at data projectors, when they come to the screen there is no login box. When they turn the projector on it is there instead, and they're not used to touch typing so they enter their password in the username box for all to see. Then when the log in, the don't know how to get their presentation from the monitor to the data projector. Default mirrored on our campus solves all of that.

It migth be doable in /etc/gnome-settings-daemon/xrandr/monitors.xml

I'm looking for a site-wide default for this, regardless of the identifier of the externally connected projector or monitor, which is not the same in every venue.

It's not a bug report, it's a feature request.