Then Itachi called his Genjutsu useless because it was a Kagebunshin. No wonder he noticed it was a Kagebunshin when his Genjutsu didn't have any effect.
Grave
misinterpretation, which is the root of an unfortunately quite common misbelief.
Kage Bunshin are just as susceptible to Genjutsu as their creator; displayed not just when Kakashi's Bunshin was, unmistakably, caught within Itachi's realm [
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], but further when Hiruzen's Bunshin were completely robbed of their vision by Hashirama's Koku Angyou [
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]. The marginal difference is that Itachi's intention was mental torture, which apparently doesn't affect Bunshin. That is what he commented on. It's not the Genjutsu
itself which was ineffective; the
outcome was. Had Itachi's aim been paralysis, as inflicted by Kasegui no Jutsu [
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], the Bunshin
would have been unable to move.