More specifically, is there a way to easily duplicate a (non linked/single user) hair particle system onto the same parent mesh, including combed guide hairs and particle settings?
Simple duplication is out of the question since the “make single user copy” button is grayed out on edited hair without first freeing the edit. The only work around I know of is a real pain. It involves creating a duplicate .blend file, and appending from it a copy of the same hair object. This will force the duplicate ParticleSettings to become ParticleSettings.001, effectively circumventing the grayed out “make single user copy”.
The “copy active to selection” feature can then optionally be used to bring the duplicated settings back to the same hair mesh.
I ask because when dealing with hair, I will often create my primary hair system, then create duplicates so that I can create variation in the settings.