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English definition of “which is which”

which is which

used in expressions that relate to being able to see the difference between two very similar things or people : For the first few months the babies looked so alike I couldn't tell which was which.Different and difference Grammar:Questions: interrogative pronouns (what, who)We use interrogative pronouns to ask questions. They are: who, which, whom, what and whose. These are also known as wh-words. Questions using these are called wh-questions:Grammar:Interrogative pronouns: usesWe use who and whom on their own:
(Definition of which is which from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)

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