this is the task for the Auto Mirror addon by @Lapineige
My request; https://github.com/lapineige/Blender_add-ons/issues/1#issuecomment-286167183
Original Code: https://github.com/lapineige/Blender_add-ons/blob/master/AutoMirror/AutoMirror_V2-4.py
French forum link: http://blenderlounge.fr/addon-auto-mirror/
This addon keeps appearing in many places, being widely used by users & addons devs. To be honest, if there was one new addon to be enabled by default, it would be this one.