Best guess layering probably in Adobe after effects shoot this then offset the clips adjusting the transparency with a mask layer so you only see the layering in some areas like hands and feet and repeating the clip several times with a slight delay each time. Thus giving the illusion there are multiple of her.
If you wish to know more I would start with learning about self clones, or using two shots of yourself (or the same actor(ess)) interacting in the same shot. Good example of this in film is Lindsey Lohan's the Parent Trap.
In this she play's two little girls who were sisters that never met; their parents separated before they could. Long story short and without too many spoilers if you haven't seen it; they meet at a camp and interact with each other. The great part is it is so believable; that these two little girls are twins, but in fact it is one actress.
So anyways I hope that puts you on the right track good luck!
Justin
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