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.
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.