1
general::
Take someone or something for someone or something else, as in I'm sorry, I mistook you for her sister, or Don't mistake that friendly smile for good intentions; he's a tough competitor. [c. 1600]
American Heritage Idioms
2
general::
Phrase(s): mistake something for something else [and] mix something up with something else
to confuse two things with each other. • Please don’t mix this idea up with that one. • I mistook my book for yours., Phrase(s): mistake someone for someone else [and] mix someone up with someone else
to confuse someone with someone else; to think that one person is another person. • I’m sorry. I mistook you for John. • Tom is always mistaking Bill for me. We don’t look a thing alike, though. • Try not to mix Bill up with Bob, his twin.
McGrawhill's American Idioms And Phrasal Verbs