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.
See also: keep, mind, more, straight
Farlex Dictionary of Idioms.

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.
See also: keep, people, straight
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs.
See also:
  • keep (two or more people) straight (in one's mind/head)
  • gaydar
  • know which is which
  • tell which is which
  • AFAICR
  • as far as I (can) recall/remember
  • it escapes (one)
  • tell (someone or something) apart
  • tell apart
  • tell people or things apart