Is it possible to show the same person multiple times on live transmission?
I'm referring to overlaying an existing video record over what's filmed live.
Do you know of any examples? Or it has to be all recorded?
Yes, this is totally possible as long as you have the ability to do a key. You can either do a luma, chroma or alpha key. It's the same as when the weatherman does the weather, but you switch the sources.
You would record whatever you wanted to be able to lay over top of the stream with either a dark background, a mono-color background, or a rotoscoped mask (alpha channel) (produced in a program like After Effects). You could then key that video over top of the live video using a video mixer and stream the result.
Caution: I have never tried this but as a Tech it makes total sense for me to recommend you try it at least at a store or venue where you test it out first to be sure you have the right duck.
A video mixer would allow you to take recorded material and live material and combine them into one video signal to feed to your device that streams video.
1) interface connections from recorded source and live camera source should be consider carefully.
2) video encoding of recorded source and live source should be considered if not tested before purchasing. SD, HD, Codecs, Containers, etc.
3) video mixers can be expensive: price ranges in the ballpark of $500 to $5000