What does the double portion of the Spirit mean?

What is the analogy of the spirit double?

There is no difficulty in spirits assuming whatever form they need for the purpose of the moment. They cannot assume permanently any disguise, but they can make themselves appear for the time what they think they wish to be. The subtle thought in spiritual life is as an artist not merely in colour or marble, but to all apparent semblance in the actual person. 

The double only reproduces itself.

Spirits have no need to induce materialisations for their own purposes, but use this thought-creation and body forth the visual tangible appearance with which one is familiar.