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Its simple. Whenever a time traveller passes through to our time , they create a new existence hence our future is not necessarily his future anymore. Of course getting back to his own future is problematic and some sort of dimensional marker may need to be created. This also avoids the grandfather paradox, he can kill as many of his ancestors as he likes, he won't disappear.
But it does make you wonder where are all these guys from the future???
Rod
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