keep people straight
keep (two or more people) straight (in one's mind/head)
To be able to remember or distinguish two or more different people clearly. A: "No, Mark is my friend from college. Marvin is the guy I work with." B: "Oh yeah, sorry, I can never keep them straight in my head." My wife has so many friends that it's hard to keep them all straight in my mind! The only way I can keep my students straight at the beginning of the year is to use a seating chart.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.
keep people straight (in one's mind)
Fig. to correctly distinguish one person from other people. The twins look exactly alike. Not even their mother can keep them straight.
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.