reference previously recorded video while still recording?

I am trying to set up a gallery installation where in a surveillance camera is set up to capture video of people moving about the gallery. I want to project the video in real time. In addition to the real time projection I'd like to reference footage from just moments before and have that project as well, creating an accumulation effect. That is to say, a person might enter the gallery and walk across the room, viewing themselves doing this in real time. After a few moments staring at their own projected image they would see their image enter the gallery space and walk across the room again, etc.

Hopefully that makes sense. I have seen other artists do similar work (see David Rokeby "Seen"), but I am not exactly sure where to start. Does anyone know of a thread or resource that covers this technique? Or does anyone just know how to do this?

Any assistance is greatly appreciated!