which is which
which is which {n. phr.}
Which is one person or thing and which is the other; one from another; what the difference is between different ones; what the name of each one is.
Joe's coat and mine are so nearly alike that I can't tell which is which.
Mr. Hadley hadn't seen his friend's daughters in such a long time that he couldn't remember which was which.
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Source: A Dictionary of American Idioms
