Naruto ep 130 , 3-4 mins in , sasuke tells naruto he can tell which 1 is real
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Also there you go , words right out itachi's mouth
That was a bombastic statement, it was meant to demoralise, give the impression of itachi being an invincible villain which is what he wanted to portray. Or are you trying to say everything each character says is true? Then you should also believe everything itachi said about he left sasuke alive to test his limits is true and thats why he murdered the clan, and not that it was a mission by anbu.
Lots of times this has happened in manga, e.g zetsu says about ametarsu:
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He says they continue to burn until their target is eradicated BUT:
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Despite being hit, Tobi survives.
A statement has to be viewed in context.
Anyway that still does'nt prove how a sharingan user can detect a shadow clone.