Episode 8 - Quintessential Quintuplets ∬ - Anime News Network
If there's one gag the Quints anime really, really needed to drop a long time ago, it's Futaro not being able to tell the girls apart. First and foremost, it's a joke that absolutely does not survive the move to animation. Even when the sisters aren't being drawn with distinct hair colors and styles, it's immediately obvious to the audience because we can presumably differentiate between voices, making Futaro's failure annoying rather than endearing. For two, it's just a really tired gag at this point, and I was already not looking forward to an entire storyline constructed around it. Needle...
Source: animenewsnetwork.com