I'm east asian n i hv trouble telling apart members of groups tat i dont stan sometimes. Even tho ITZY's ryujin n yuna dont look alike, i cant tell sometimes who is who cos of not paying attention to their differences. I might be getting better at it with having to pay attention now cos of reasons, tho...
Otoh, many SONEs hv historically difficulty telling apart the so-called triplets cos of their similarity n the sheer number (9) of members, angles n hairstyles(?), face shapes from way back in history(2007 onwards); i too found it hard to tell apart when i started stanning SNSD in 2018, watching/looking at videos/pix from 2007-2018. It got easier the l8r the era cos they grew differently, hair styles, face shapes n fashion styling. The triplets? Yuri, YoonA n Seohyun. Even now if i come across them from olden times, sometime i only recognise them cos i memorised their appearance n not becos its obvious. If tat makes any sense. So yeah, its hard to tell certain idols apart. Its partly familiarity n experience. Idk how to explain it any better than tat.