Dear everyone who is throwing all their bets on the girl on the left being the “real Arya” simply because she’s dressed just like her:
There’s a pretty clear cut fashion trend in Braavos.
Those “Oysters, clams, and cockles!” girls seem to have a uniform. They all wear their hair in a very similar fashion and their outfits are nearly identical. In fact, some of them are identical to Arya’s adopted look.
Remember that Arya was tasked with blending seamlessly into her role, not standing out. There’s no reason she’s looking for individuality in dress or hair. The literal point was to be just like the others.