well, you can't make anyone do anything first off. But you can tell them that there is more then one of you. After that, it's whatever they believe unless you present results
That's more like superhero ability stuff...I mean I bet it's possible, but something like that would be found under 'illusion control' in the comic book world. Which basically means it's super-advanced and would take tons and tons of learning and practice. Not to mention it really doesn't serve a useful purpose other than negative things like combat. If you're asking all of this to tease your sister...well, I won't bark down your throat but please try to tease her like a normal brother. Please?
On a more technical note, illusion control has two possible forms: Tangible illusions and non-tangible. Tangible is basically rediculous and improbable, because it's more magic than anything else, as you would have to create fake matter that reflects light in more or less exactly the same way as a normal 'real' object. Non-tangible illusions, however, are more probable, since it doesn't really create a 'physical' illusion. Instead, you 'trick' the victim into thinking there is another of something, which is basically controlling their minds to see certain things. Like that damn wheelchair kid in X-men 2. He was annoying. And damned powerful...anyway yeah take what you will from that.
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i guess it coupd be possible by projecting a psychic impression onto the targets mind that TELLS them that they are seing more than one when infact they arent :/
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